Ophthalmic instrument having adaptive optic subsystem with multiple stage phase compensator

ABSTRACT

An improved ophthalmic instrument including a wavefront sensor that estimates aberrations in reflections of the light formed as an image on the retina of the human eye. A phase compensator, operably coupled to the wavefront sensor, spatially modulates the phase of incident light to compensate for the aberrations estimated by the wavefront sensor. The phase compensator includes a first stage and at least one other additional stage, wherein the first stage compensates for a defocus component of the aberrations, and the additional stage(s) compensate for other higher order components of the aberrations. The first stage preferably comprises a variable focus lens and the additional stage(s) preferably comprise a deformable mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present Application is related to the following United StatesPatent Applications: copending application Ser. No. ______ (AttorneyDocket No. 108-109USA000), filed concurrently herewith, entitled“Ophthalmic Instrument With Adaptive Optic Subsystem That MeasuresAberrations (Including Higher Order Aberrations) Of A Human Eye And ThatProvides A View Of Compensation Of Such Aberrations To The Human Eye,”by Bruce M. Levine, Allan Wirth, and C. Harry Knowles; copendingapplication Ser. No. ______ (Attorney Docket No. 108-130USA000), filedconcurrently herewith, entitled “Ophthalmic Instrument Having WavefrontSensor With Multiple Imaging Devices that Simultaneously CaptureMultiple Images Of An Array Of Spots Produced By A Lenslet Array,” byAllan Wirth; copending application Ser. No. ______ (Attorney Docket No.108139USA000) entitled “Ophthalmic Instrument Having Hartmann WavefrontSensor With Extended Source” by Allan Wirth; copending application Ser.No. ______ (Attorney Docket No. 108-140USA000) entitled “OphthalmicInstrument Having Hartmann Wavefront Sensor Deriving Location Of SpotsWith Spot Fitting Techniques” by Allan Wirth; copending application Ser.No. 09/874,403, filed Jun. 5, 2001, entitled “Ophthalmic ImagingInstrument Having An Adaptive Optical Subsystem That Measures PhaseAberrations in Reflections Derived From Light Produced By An ImagingLight Source And That Compensates For Such Phase Aberrations WhenCapturing Images of Reflections Derived From Light Produced By The SameImaging Light Source,” by Bruce M. Levine; copending application Ser.No. 09/874,401, filed Jun. 5, 2001, entitled “Modular Adaptive OpticalSubsystem for Integration With A Fundus Camera Body and CCD Camera Unitand Improved Fundus Camera Employing Same,” by Bruce M. Levine;copending application Ser. No. 09/874,404, filed Jun. 5, 2001, entitled“Ophthalmic Instrument Having An Integral Wavefront Sensor and DisplayDevice That Displays A Graphical Representation of High OrderAberrations of the Human Eye Measured by the Wavefront Sensor,” by BruceM. Levine; and copending application Ser. No. 09/874,903, filed Jun. 5,2001, entitled “Ophthalmic Instrument Having An i Integral WavefrontSensor and Display Device That Displays A Graphical Representation ofHigh Order Aberrations of the Human Eye Measured by the WavefrontSensor,” by Bruce M. Levine, each being assigned to Adaptive OpticsAssociates, Inc., and incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to ophthalmic instruments that areused to examine or treat the eye, including ophthalmic examinationinstruments (such as phoropters and autorefractors) that measure andcharacterize the aberrations of the human eye in order to prescribecompensation for such aberrations via lens (such as glasses or contactlens) or surgical procedure (such as laser refractive surgery), inaddition to ophthalmic imaging instruments (such as fundus cameras,corneal topographers, retinal topographers, corneal imaging devices, andretinal imaging devices) that capture images of the eye.

[0004] 2. Summary of the Related Art

[0005] The optical system of the human eye has provided man with thebasic design specification for the camera. Light comes in through thecornea, pupil and lens at the front of the eye (as the lens of thecamera lets light in). This light is then focused on the inside wall ofthe eye called the retina (as on the film in a camera). This image isdetected by detectors that are distributed over the surface of theretina and sent to the brain by the optic nerve which connects the eyeto the brain (as film captures the image focused thereon).

[0006]FIG. 1 shows a horizontal cross section of the human eye. The eyeis nearly a sphere with an average diameter of approximately 20 mm.Three membranes-the cornea and sclera outer cover, the choroid and theretina—enclose the eye. The cornea 3 is a though transparent tissue thatcovers the anterior surface of the eye. Continuous with the cornea 3,the sclera 5 is an opaque membrane that encloses the remainder of theeye. The choroid 7 lies directly below the sclera 5 and contains anetwork of blood vessels that serves as the major source of nutrition tothe eye. At its anterior extreme, the choroid 7 includes a ciliary body9 and an iris diaphragm 11. The pupil of the iris diaphragm 11 contractsand expands to control the amount of light that enters the eye.Crystalline lens 13 is made up of concentric layers of fibrous cells andis suspended by fibers 15 that attach to the ciliary body 9. Thecrystalline lens 13 changes shape to allow the eye to focus. Morespecifically, when the ciliary muscle in the ciliary body 9 relaxes, theciliary processes pull on the suspensory fibers 15, which in turn pullon the lens capsule around its equator. This causes the entire lens 13to flatten or to become less convex, enabling the lens 13 to focus lightfrom objects at a far away distance. Likewise, when the ciliary muscleworks or contracts, tension is released on the suspensory fibers 15, andsubsequently on the lens capsule, causing both lens surfaces to becomemore convex again and the eye to be able to refocus at a near distance.This adjustment in lens shape, to focus at various distances, isreferred to as “accommodation” or the “accommodative process” and isassociated with a concurrent constriction of the pupil.

[0007] The innermost membrane of the eye is the retina 17, which lies onthe inside of the entire posterior portion of the eye. When the eye isproperly focused, light from an object outside the eye that is incidenton the cornea 3 is imaged onto the retina 17. Vision is afforded by thedistribution of receptors (e.g., rods and cones) over the surface of theretina 17. The receptors (e.g., cones) located in the central portion ofthe retina 17, called the fovea 19 (or macula), are highly sensitive tocolor and enable the human brain to resolve fine details in this area.Other receptors (e.g., rods) are distributed over a much larger area andprovides the human brain with a general, overall picture of the field ofview. The optic disc 21 (or the optic nerve head or papilla) is theentrance of blood vessels and optic nerves from the brain to the retina17. The inner part of the posterior portion of the eye, including theoptic disc 21, fovea 19 and retina 17 and the distributing blood vesselsin called the ocular fundus 23. Abnormalities in the cornea andcrystalline lens and other portions of the eye contribute to refractiveerrors (such as defocus, astigmatism, spherical aberrations, and otherhigh order aberrations) in the image captured by the retina.

[0008] A phoropter (or retinoscope) is an ophthalmic instrument thatsubjectively measures the refractive error of the eye. A typicalphoropter consists of a pair of housings in which are positionedcorrective optics for emulating the ophthalmic prescription required tocorrect the vision of the patient whose eyes are being examined.Typically, each housing contains sets of spherical and cylindricallenses mounted in rotatable disks. The two housings are suspended from astand or wall bracket for positioning in front of the patient's eyes.Further, in front of each refractor housing a number of accessories aremounted, typically on arms, so that they may be swung into place beforethe patient's eyes. Typically, these accessories include a variablepower prism known as a Risley prism, Maddox rods, and a cross cylinderfor performing the Jackson cross cylinder test. In determining apatient's distance prescription, the patient views a variety of alphanumeric characters of different sizes through various combinations ofthe spherical and/or cylindrical lenses supported in the refractorhousings until the correct prescription is emulated. The characters,which are typically positioned 6 meters away, may be on a chart or maybe projected on a screen by an acuity projector. For near vision testingthe same procedure is repeated, expect that the alpha numeric charactersviewed by the patient are positioned on a bracket 20 to 65 centimetersin front of the refractor housing. The cross cylinder is used to refinethe power and axis position of the cylindrical component of thepatient's prescription. The cross cylinder is a lens consisting of equalpower plus and minus cylinders with their axes 90 degrees apart. It ismounted in a loupe for rotation about a flip axis which is midwaybetween the plus and minus axes.

[0009] An autorefractor is an ophthalmic instrument that quantitativelymeasures the refractor errors of the eye. Light from an illuminationsource (typically an infra-red illumination source) is directed into theeye of the patient being examined. Reflections are collected andanalyzed to quantitatively measure the refractive errors of the eye.

[0010] Conventional phoropters and autorefractors characterize therefractive errors of the eye only in terms of focal power (typicallymeasured in diopter) required to compensate for such focal errors; thus,such instruments are incapable of measuring and characterizing thehigher order aberrations of the eye, including astigmatism and sphericalaberration. Examples of such devices are described in the following U.S.Pat. Nos. 4,500,180; 5,329,322; 5,455,645; 5,629,747; and 5,7664,561.

[0011] Instruments have been proposed that utilize wavefront sensors tomeasure and characterize the high order aberrations of the eye. Forexample, U.S. Pat. No. 6,007,204, to Fahrenkrug et al. discloses anapparatus for determining refractive aberrations of the eye wherein asubstantially collimated beam of light is directed to the eye ofinterest. This collimated light is focused as a secondary source on theback of the eye, thereby producing a generated wavefront that exits theeye along a return light path. A pair of conjugate lenses direct thewavefront to a microoptics array of lenslet elements, where incrementalportions of the wavefront are focuses onto an imaging substrate.Deviation of positions of the incremental portions relative to a knownzero or “true” position (computed by calculating the distance betweenthe centroids of spots formed on the imaging substrate by the lensletarray) can be used to compute refractive error relative to a known zeroor ideal diopter value. Because the optical power at the lenslet doesnot equal the optical power of the measured eye, the optical power ofthe lenslet is corrected by the conjugate lens mapping function tointerpolate the power of the eye. This refractive error is reported tothe user of the apparatus through an attached LCD.

[0012] In U.S. Pat. Nos. 5,777,719; 5,949,521; and 6,095,651, Williamsand Liang disclose a retinal imaging method and apparatus that producesa point source on a retina by a laser. The laser light reflected fromthe retina forms a distorted wavefront at the pupil, which is recreatedin the plane of a deformable mirror and a Shack-Hartmann wavefrontsensor. The Shack-Hartmann wavefront sensor includes an array oflenslets that produce a corresponding spot pattern on a CCD camera bodyin response to the distorted wavefronts. Phase aberrations in thedistorted wavefront are determined by measuring spot motion on the CCDcamera body. A computer, operably coupled to the Shack-Hartmannwavefront sensor, generates a correction signal which is fed to thedeformable mirror to compensate for the measured phase aberrations.After correction has been achieved via the wavefront sensing of thereflected retinal laser-based point source, a high-resolution image ofthe retina can be acquired by imaging a krypton flash lamp onto theeye's pupil and directing the reflected image of the retina to thedeformable mirror, which directs the reflected image onto a second CCDcamera body for capture. Examples of prior art Shack-Hartmann wavefrontsensors are described in U.S. Pat. Nos. 4,399,356; 4,725,138, 4,737,621,and 5,529,765; each herein incorporated by reference in its entirety.

[0013] Notably, the apparatus of Fahrenkrug et al. does not provide forcompensation of the aberrations of the eye. Moreover, the apparatus ofFahrenkrug et al. and the apparatus of Williams and Liang do not providea view of the compensation of the aberrations to the eye. Thus, thepatient cannot provide immediate feedback as to the accuracy of themeasurement; and must wait until compensating optics (such as a contactlens or glasses that compensate for the measured aberrations) areprovided in order to provide feedback as to the accuracy of themeasurement. This may lead to repeat visits, thereby adding significantcosts and inefficiencies to the diagnosis and treatment of the patient.

[0014] In addition, the wavefront sensing apparatus (i.e., the lensletarray and imaging sensor) of Fahrenkrug et al. and of Williams and Liangare susceptible to a dot crossover problem. More specifically, in ahighly aberrated eye, the location of spots produced on the imagingsensor may overlap (or cross). Such overlap (or crossover) introduces anambiguity in the measurement that must be resolved, or an error will beintroduced.

[0015] In addition, the signal-to-noise ratio provided by traditionalHartmann sensing techniques in measuring the aberrations of the humaneye is limited, which restricts the potential usefulness of ophthalmicinstruments that embody such techniques in many real-world ophthalmicapplications. More specifically, the basic measurement performed by anyHartmann wavefront sensor is the determination of the locations of theHartmann spots. Traditionally, this has been done by calculating thecentroid of the illumination in a pixel subaperture defined around eachspot.

[0016] Centroid calculation is conceptually very simple. To calculatethe centroid of the light distribution in the x-direction, weights areassigned to each column of pixels in the pixel subaperture and themeasured intensity for each pixel in the pixel subaperture is multipliedby the weight corresponding to the column of the given pixel and summedtogether. If the weights vary linearly with the distance of the columnfrom the center of the pixel subaperture, this sum will be a measure ofthe x-position of the light distribution. The sum needs to be normalizedby dividing by the sum of the unweighted intensities. To calculate thecentroid of the light distribution in the y-direction, weights areassigned to each row of pixels in the pixel subaperture and the measuredintensity for each pixel in the pixel subaperture is multiplied by theweight corresponding to the row of the given pixel and summed together.If the weights vary linearly with the distance of the column from thecenter of the pixel subaperture, this sum will be a measure of they-position of the light distribution. The sum needs to be normalized bydividing by the sum of the unweighted intensities. Such centroidcalculation may be represented mathematically as follows:$x_{c} = \frac{\sum\limits_{i}{\sum\limits_{j}{w_{j}*I_{ij}}}}{\sum\limits_{i}{\sum\limits_{j}I_{ij}}}$$y_{c} = \frac{\sum\limits_{i}{\sum\limits_{j}{w_{i}*I_{ij}}}}{\sum\limits_{i}{\sum\limits_{j}I_{ij}}}$

[0017] where i and j identify the rows and columns, respectively, of thepixel subaperture; w_(i) and w_(j) are the weights assigned to givenrows and columns, respectively, of the pixel subaperture; and I_(ij) isthe intensity of a given pixel in row i and column j of the pixelsubaperture.

[0018] This “center-of-light” measurement is analogous to the usualcenter-of-mass calculation. FIG. 2 shows a one dimensionalrepresentation of the intensity distribution on a row of detector pixelsand a set of weights. These weights are simply the distance of thecenter of the pixel from the center of the pixel subaperture in units ofpixel spacing.

[0019] However, centroid calculation is disadvantageous because it issusceptible to background noise and thus may be unacceptable in manyreal-world environments where background noise is present. FIG. 2reveals these shortcomings. Note that the highest weights are applied topixels farthest from the center. Note also that, typically, there isvery little light in these regions. This means that the onlycontribution to these highly weighted pixels comes from background lightand noise. Because of the high weight, these pixels adversely affect theaccuracy of the measurement. As the size of the pixel region of thatmeasures such spot motion is increased to provide greater tilt dynamicrange, the noise problem is made worse by increasing the number ofpixels that usually have no useful signal.

[0020] An even larger problem stems from the centroid algorithmssensitivity to residual background signal. Consider a pixel region thatis 10×10 pixels in size. Typically, a given spot will occupy less than10 of those pixels. Suppose there is a residual background signal thatproduces, per pixel, 1% of the signal from the given spot. Because it ispresent in all 100 pixels, its contribution to the total signal is equalto that of the spot. Even if this background is perfectly uniform, whenthe centroid is normalized by the total signal, that divisor will betwice its true size. This will make the calculated centroid half iscorrect value. If the background is not uniform, its effect on thecentroid can easily overwhelm that of the spot. Thus, the susceptibilityof the centroid algorithm to background noise makes it unacceptable inmany real-world environments where such background noise is present.

[0021] Thus, there is a great need in the art for improved ophthalmicinstruments that measure and characterize the aberrations of the humaneye in a manner that avoids the shortcomings and drawbacks of prior artophthalmic instruments.

SUMMARY OF THE INVENTION

[0022] Accordingly, a primary object of the present invention is toprovide improved ophthalmic instruments that measure and characterizethe aberrations of the human eye in a manner free of the shortcomingsand drawbacks of prior art ophthalmic instruments.

[0023] Another object of the present invention is to provide anophthalmic instrument that measures the aberrations (including higherorder aberrations) of the eye(s) of a patient and provides the patientwith a view of correction (e.g., compensation) of the measuredaberrations such that the patient can provide instant feedback as to theaccuracy of the measurement.

[0024] Another object of the present invention is to provide anophthalmic instrument that includes an adaptive optic subsystem thatmeasures the aberrations (including higher order aberrations) of theeye(s) of a patient, and an internal fixation target, operably coupledto the adaptive optic subsystem, to provide the patient with a view ofcorrection (e.g., compensation) of the measured aberrations such thepatient can provide instant feedback as to the accuracy of themeasurement.

[0025] Another object of the present invention is to provide anophthalmic instrument that includes a wavefront sensor that measures theaberrations (including higher order aberrations) of the eye(s) of apatient, an internal fixation target and phase compensator that providesthe patient with a view of correction (e.g., compensation) of themeasured aberrations, and high resolution image capture capabilities.

[0026] Another object of the present invention is to provide anophthalmic instrument that provides more efficient and effectiveprescription of corrective optics (e.g., classes or contact lens) bymeasuring the aberrations (including higher order aberrations) of theeye(s) of a patient, identifying a set of prescriptions that correspondto the measured aberrations of the eye(s), and providing the patientwith a view of correction (e.g., compensation) provided by theprescriptions in the set to thereby enable instant patient feedback andpatient selection of the best prescription (if necessary).

[0027] Another object of the present invention is to provide anophthalmic instrument that provides more efficient and effectivedispensing of corrective optics (e.g., classes or contact lens) by:measuring the aberrations (including higher order aberrations) of theeye(s) of a patient, identifying a set of corrective optics thatcorrespond to the measured aberrations of the eye(s), and providing thepatient with a view of correction (e.g., compensation) provided by thecorrective optics in the set to thereby enable the patient to select thebest corrective optic (if necessary) with minimal assistance.

[0028] Another object of the present invention is to provide a systemthat provides for efficient dispensing of glasses whose frame isoptimally fitted to the dimension of the head and face of the patientand whose corrective lens elements optimally compensate for theaberrations of eyes. The system measures the aberrations (includinghigher order aberrations) of the eyes of a patient, identifies a set oflens elements that correspond to the measured aberrations of the eyes,and provides the patient with a view of correction (e.g., compensation)provided by the lens elements in the set to enable the patient to selectthe optimal corrective lens element (if necessary). In addition, thesystem performs imaging and dimensioning analysis on the head and faceof the patient to generate a profile of the dimensions of the head andface of the patient, and identifies a set of frames that correspond tothe patient's profile to enable the patient to select one of the framesin the set. The patient selected corrective lens elements and frame(which may be custom built) are integrated into glasses and provided tothe patient.

[0029] Another object of the present invention is to provide anophthalmic instrument that includes a wavefront sensor that estimatesthe aberrations (including higher order aberrations) of the eye(s) and amulti-stage phase compensator (such as the variable focus lens (VFL) anda deformable mirror) having multiple stages that compensate fordifferent parts of the aberrations of the eye as estimated by thewavefront sensor.

[0030] Another object of the present invention is to provide anophthalmic instrument that includes a wavefront sensor that estimatesthe aberrations (including higher order aberrations) of the eye(s) and amulti-stage phase compensator comprising a variable focus lens (VFL) anda deformable mirror, wherein the variable focus lens compensates for thedefocus component of such aberrations, and the deformable mirrorcompensates for other higher order components of such aberrations.

[0031] Another object of the present invention is to provide anophthalmic instrument that includes a Hartmann style wavefront sensorthat estimates the aberrations (including higher order aberrations) ofthe eye(s) in real time in order to minimize the adverse effects of eyemovement and/or accommodation on the accuracy of such estimates, therebycapable of avoiding immobilization of the eye and/or paralysis of theeye via drugs.

[0032] Another object of the present invention is to provide anophthalmic instrument that includes a Hartmann style wavefront sensorthat estimates the aberrations (including higher order aberrations) ofthe eye(s) by calculating one or more of the following data items inreal time in order to minimize the adverse effects of eye movementand/or accommodation on the accuracy of such estimates, the data itemsincluding: the geometric reference of nominal null of the sensor,position and shape of the pupil of the eye in the local coordinatesystem of the sensor, and the pixel subapertures of the imaging deviceof the sensor that avoid dot crossover.

[0033] Another object of the present invention is to provide anophthalmic instrument that includes a Hartmann style wavefront sensorthat estimates the aberrations (including higher order aberrations) ofthe eye(s), wherein the wavefront sensor is equipped with an improvedtechnique for determining the location of the Hartmann spot in a givenpixel subaperture defined around that spot in a manner that providesbetter performance (e.g., a lower threshold signal-to-noise ratio) undersuch real-world conditions.

[0034] Another object of the present invention is to provide anophthalmic instrument that includes a Hartmann style wavefront sensorthat estimates the aberrations (including higher order aberrations) ofthe eye(s), wherein the wavefront sensor utilizes an extended source ina manner that improves the signal-to-noise ratio of the wavefrontmeasurements calculated therein.

[0035] Another object of the present invention is to provide anophthalmic instrument that includes a Hartmann style wavefront sensorthat projects an image of an extended source onto the retina of theeye(s), captures a plurality of images derived from the retinalreflections of the projected extended source, and applies imagecorrelation techniques in the digital domain to image data derived fromthe plurality of captured images in order to estimate the local tilt ofsuch retinal reflections. The local tilt estimates are reconstructed toform data representative of the aberrations (including defocus,spherical aberration, coma, astigmatism in addition to other higherorder aberrations) of such retinal reflections, which are characteristicof the aberrations of the eye(s) of the patient.

[0036] These and other objects of the present invention will becomeapparent hereinafter and in the claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] For a more complete understanding of the present invention, thefollowing Detailed Description of the Illustrative Embodiment should beread in conjunction with the accompanying Drawings.

[0038]FIG. 1 is a pictorial illustration of a horizontal cross sectionof the human eye.

[0039]FIG. 2 is one dimensional representation of the intensitydistribution on a row of detector pixels and a set of weights for use inprior art techniques for determining the centroid of such detectorpixels.

[0040]FIG. 3A is a schematic representation of the wavefront sensingcomponents of an exemplary ophthalmic instrument according to thepresent invention.

[0041]FIG. 3B is a schematic representation depicting the planarwavefront and distorted wavefront produced via reflection of a pointsource imaged onto the retina of an ideal eye and an aberrated eye,respectively.

[0042]FIG. 4 is a schematic representation of the fixation targetcomponents of an exemplary ophthalmic instrument according to thepresent invention.

[0043]FIG. 5 is a schematic representation of the imaging components ofan exemplary ophthalmic instrument according to the present invention.

[0044]FIGS. 6A and 6B are schematic representations of exemplaryembodiments of ophthalmic instruments according to the presentinvention, including wavefront sensing, an internal fixation target andhigh resolution image capture capabilities.

[0045]FIG. 6C is a schematic representation of a display viewable on thedisplay device (in addition to a keypad) of the ophthalmic instrumentsof FIGS. 6A and 6B, wherein the display includes a graphicalrepresentation of the aberrations of the human eye (including high orderaberrations of the human eye) as measured by the wavefront sensor of theophthalmic instrument.

[0046]FIGS. 7A and 7B are functional block diagrams that illustrate amulti-stage phase compensator that is embodied as part of an adaptiveoptic-based ophthalmic instrument according to the present invention.

[0047]FIGS. 8A and 8B are pictorial illustrations of a siliconmicro-machined membrane deformable mirror that may be embodied as partof the phase compensator of the adaptive optic-based ophthalmicinstrument of the present invention.

[0048]FIG. 9 is a schematic illustration of exemplary Shack-Hartmannwavefront sensing components that may be embodied within the ophthalmicinstruments of the present invention.

[0049]FIG. 10A is a functional block diagram of an exemplary embodimentof the components of imaging device 311 of FIG. 9.

[0050]FIG. 10B is a functional block diagram of an exemplary embodimentof the components of image processor 310 of FIG. 9.

[0051]FIG. 11 is a pictorial representation of the Hartmann spot patternthat is formed at approximately a lenslet focal length fL behind thelenslet array of the Shack-Hartmann wavefront sensor of FIG. 9.

[0052]FIG. 12 is a pictorial representation of an exemplary lensletarray of the Shack-Hartmann wavefront sensor of FIG. 9, including asubstantially-opaque element at the center of each lenslet of thelenslet array for use in determining the geometric reference of nominalnull for the sensor.

[0053]FIGS. 13A, 13B, 14A and 14B are pictorial illustrations ofexemplary image forming and image capture components of theShack-Hartmann wavefront sensor of FIG. 9, including a relay lens andthe imaging device mounted on a linear actuator that has sufficienttravel to allow the imaging device to image all planes from the planesubstantially near the lenslet array itself, back to the focal plane ofthe longest focal length lenslet array.

[0054]FIG. 15 is a flow chart illustrating an exemplary image processingtechniques that are applied to multiple images of the pupil image planeof the Shack-Hartmann wavefront sensor of FIG. 9 to thereby derive thegeometric reference of the wavefront sensor.

[0055]FIG. 16 is a pictorial illustration that shows the spatialposition of the system pupil (the pupil of the eye under test) in anexemplary local coordinate system used by the Shack-Hartmann wavefrontsensor of FIG. 9.

[0056]FIG. 17 is a flow chart illustrating an exemplary image processingtechnique that automatically locates the position of the system pupil(e.g., the pupil of the eye under test) in the local coordinate systemof the Shack-Hartmann wavefront sensor of FIG. 9.

[0057]FIG. 18 is a graphical illustration of exemplary slices (RC1 . . .RC8) from a centroid C to the periphery of an image of the pupil imageplane (e.g., in the u,v pixel space), which are generated in theprocessing of FIG. 17.

[0058]FIG. 19A is a flow chart that illustrates a mechanism, which ispreferably employed by the Shack-Hartmann wavefront sensor of FIG. 9,that dynamically identifies the sub-arrays (pixel areas) of the Hartmannspot imaging device (e.g., the imaging device that will be used for thedetermination of Hartmann spot positions) that avoids dot crossover fora particular wavefront measurement.

[0059]FIG. 19B is a pictorial illustration of the projection of a rayfrom a given Hartmann spot in the spot image plane to the plane of thelenslet array of the Shack-Hartmann wavefront sensor of FIG. 9, which isused in the processing of FIG. 19A.

[0060]FIG. 20A illustrates an improved Shack-Hartmann wavefront sensinghead of an ophthalmic instrument according to the present invention,wherein fiducial points of the lenslet array are used to provide thegeometric reference of nominal null and the delays associated withcapture of the required multiple images are avoided. The improvedShack-Hartmann wavefront sensing head includes a relay lens, beamsplitter and multiple imaging devices that cooperate to capture imagesof the fiducial point image plane and the Hartmann spot imaging plane inreal time in order to minimize the adverse effects of eye movementand/or accommodation on wavefront measurements performed therein.

[0061]FIG. 20B illustrates an improved Shack-Hartmann wavefront sensinghead of an ophthalmic instrument according to the present invention,wherein the image processing techniques on multiple images of the pupilimage plane are used to derive the geometric reference to nominal null(as described above with respect to FIG. 15) and the delays associatedwith capture of the required multiple images are avoided. The improvedShack-Hartmann wavefront sensing head includes a relay lens, beamsplitter and multiple imaging devices that cooperate to capture imagesof the pupil image plane and the Hartmann spot imaging plane in realtime in order to minimize the adverse effects of eye movement and/oraccommodation on wavefront measurements performed therein.

[0062]FIGS. 20C and 20D illustrate improved Shack-Hartmann wavefrontsensing heads of ophthalmic instruments according to the presentinvention, wherein the operations of FIG. 19 are used to dynamicallyidentify the sub-arrays (pixel areas) of the Hartmann spot imagingdevice (e.g., the imaging device that will be used for the determinationof Hartmann spot positions) that dot crossover for a particularwavefront measurement, and the delays associated with the capture of therequired multiple images are avoided. The improved wavefront sensingheads include a beam splitter and multiple imaging devices thatcooperate to capture multiple images of different planes between thelenslet array itself and the focal plane of the lenslet array asrequired by the operations of FIG. 19 in real time in order to minimizethe adverse effects of eye movement and/or accommodation on wavefrontmeasurements performed therein.

[0063] FIGS. 21A-21C are pictorial illustrations of exemplary Hartmannwavefront sensors.

[0064]FIG. 22 is a flow chart illustrating an improved technique(embodied within a Hartmann wavefront sensor and ophthalmic instrumentutilizing such a sensor) that determines the location of the Hartmannspot in a given pixel subaperture defined around that spot in a mannerthat provides better performance (e.g., a lower thresholdsignal-to-noise ratio) under such real-world conditions.

[0065]FIG. 23 is a flow chart illustrating exemplary operations of anophthalmic instrument that provides more efficient and effectiveprescription of corrective optics (e.g., classes or contact lens) bymeasuring the aberrations (including higher order aberrations) of theeye(s) of a patient, identifying a set of prescriptions that correspondto the measured aberrations of the eye(s), and providing the patientwith a view of correction (e.g., compensation) provided by theprescriptions in the set to thereby enable instant patient feedback andpatient selection of the best prescription (if necessary).

[0066]FIG. 24A is a pictorial illustration of a system that providesmore efficient and effective dispensing of corrective optics (e.g.,classes or contact lens) by: measuring the aberrations (including higherorder aberrations) of the eye(s) of a patient, identifying a set ofcorrective optics that correspond to the measured aberrations of theeye(s), and providing the patient with a view of correction (e.g.,compensation) provided by the corrective optics in the set to therebyenable the patient to select the optimal corrective optic (if necessary)with minimal assistance. The system preferably includes an imaging anddimension subsystem that generates a profile of the dimensions (and/orother relevant spatial characteristics) of the face and head of thepatient. A set of frames that correspond to the patient's profile areidentified to enable the patient to select one of the frames in the set.The patient selected corrective optics and frame (which may be custombuilt) are integrated into glasses and provided to the patient, therebyproviding the patient with a frame that is optimally fitted to thedimension of the patient's head and face and with corrective optics thatoptimally compensate for the aberrations of patient's eyes.

[0067]FIG. 24B is a flow chart that illustrates the operations of thesystem of FIG. 24A that provides the dispensing of corrective optics(e.g., glasses or contact lens) with minimal human assistance to thepatient.

[0068]FIG. 25A is a schematic illustration of typical Hartmann wavefrontsensors.

[0069]FIG. 25B is a schematic illustration of an improved Hartmannwavefront sensor for use in an ophthalmic instrument according to thepresent invention, which includes an extended source that improves thesignal-to-noise ratio of the wavefront measurements calculated therein.

[0070]FIG. 26 is a functional block diagram that illustrates imagecorrelation techniques in the digital domain that are applied to theimage data that represents an image of the extended source to estimatethe local tilt of the incident wavefront over a subaperture of thesensor. This technique is applied to the image data for each image ofthe extended source (for the plurality of images of the extended sourcethat are formed by the subapertures of the sensor) to estimate the localtilt of the incident wavefront over the subapertures of the sensor.

[0071]FIGS. 27A and 27B are schematic representations of exemplaryophthalmic instruments that embody the improved Hartmann wavefrontsensor of FIG. 25B according to the present invention. The ophthalmicinstruments project an image of an extended source onto the retina ofthe eye(s), capture a plurality of images of the extended source(derived from the retinal reflections of the projected extended sourcethat are formed by the subapertures of the sensor), and apply imagecorrelation techniques in the digital domain to image data derived fromthe plurality of images of the extended source in order to estimate thelocal tilt of such retinal reflections. The local tilt estimates arereconstructed to form data representative of the aberrations (includingdefocus, spherical aberration, coma, astigmatism in addition to otherhigher order aberrations) of such retinal reflections, which arecharacteristic of the aberrations of the eye(s) of the patient. Theophthalmic instrument of FIG. 27B provides wavefront sensing, aninternal fixation target, and high resolution image capture capabilitiesaccording to the present invention.

DETAILED DESCRIPTION OF THE BEST MODE EMBODIMENTS OF THE INVENTION

[0072] Referring to the figures in the accompanying Drawings, thepreferred embodiments of the ophthalmic instruments of the presentinvention will be described in greater detail, wherein like elementswill be indicated using like reference numerals.

[0073] According to the present invention, an ophthalmic instrumentincludes an adaptive optic subsystem that forms an image of a wavefrontsensing illumination source on the retina of the eye under examination,which is reflected (thereby exiting the pupil of the eye as distortedwavefronts) and directed back to the instrument. An image of thereflected wavefronts (which represent retroreflection of the imageformed on the retina and exit the pupil of the eye as distortedwavefronts) is created on a phase compensator (which preferablycomprises a variable focus lens and a deformable mirror) and recreatedat a wavefront sensor. The phase compensator operates to spatiallymodulate the phase of the image of the distorted wavefronts incidentthereon. The wavefront sensor measures the phase aberrations in thewavefronts incident thereon and operates in a closed-loop fashion with acontroller to control the phase compensator to compensate for such phaseaberrations to restore the distorted wavefronts to phase-alignedwavefronts, which are directed to the wavefront sensor (for furtherwavefront measurement and compensation if required). In this manner, thewavefront sensor and phase compensator compensate for the phaseaberrations of the eye under examination. The aberrations of thedistorted wavefront measured by the wavefront sensor are characteristicof the aberrations of the eye. The wavefront sensor is preferablyoperably coupled to a display device that generates a graphicalrepresentation (such as a wavefront map that depicts the OPD over thepupil, e.g., subapertures, of the wavefront sensor, or a graphicaldisplay of the coefficients of the OPD function) of the aberrations ofthe eye as measured by the wavefront sensor.

[0074] Concurrently therewith, an image of an internal fixation targetis created at the phase compensator, which operates to spatiallymodulate the phase of the image of the fixation target incident thereonto compensate for the aberrations of the eye under examination. Thephase compensated image of the fixation target produced by the phasecompensator is created at the pupil of the eye under examination. Thisoperation provides the patient with a view of correction (e.g.,compensation) of the aberrations of the eye under examination, such thepatient can provide instant feedback as to the accuracy of themeasurement.

[0075] In addition, the ophthalmic instrument may perform imagingoperations whereby light from an imaging illumination source is directedonto the pupil of the eye, which is reflected and directed back to theinstrument. An image of these reflections is created on the phasecompensator, which operates to spatially modulate the phase of thisimage to compensate for the aberrations of the eye under examination. Animaging device captures an image of the phase-aligned reflections outputfrom the phase compensator. This operation provides the capture (andsubsequent processing and display) of high-resolution images of the eyeunder examination.

[0076] As described herein, the present invention is broadly applicableto (and can be embodied within) ophthalmic examination instruments thatcharacterize the optical aberrations of the eye, such as phoropters andautorefractors. In addition, other aspects of the present invention arebroadly applicable to (and can be embodied within) any ophthalmicinstrument that is used to examine or treat the eye, includingophthalmic examination instruments (such as phoropters andautorefractors) and ophthalmic imaging instruments that capture imagesof the eye (such as fundus cameras, corneal topographers, retinaltopographers, corneal imaging devices, and retinal imaging devices).

[0077] Referring now to FIG. 3A, there is shown, in schematic form, thewavefront sensing components of ophthalmic instrument according to thepresent invention. As shown, the wavefront sensing components include awavefront sensing illumination source 51 (e.g., a ring of infrared laserdiodes with a characteristic wavelength, for example, of 780 nm) thatcooperates with optical elements 59 to form an image of the wavefrontsensing illumination source 51 on the retina of the eye 1, which isreflected (and exits the pupil of the eye as distorted wavefronts) anddirected back to the instrument.

[0078] The light produced from the wavefront sensing illumination source51 forms substantially planar (e.g., phase-aligned) wavefronts that aredirected to the pupil of the eye. These planar wavefronts are imagedonto the retina of the eye by the crystalline lens. The image formed onthe retina may be a point source image. Alternatively, as describedbelow with respect to FIGS. 25B, 26, 27A and 27B, the image formed onthe retina may be an extended source image.

[0079] As illustrated in FIG. 3B, the light reflected from the retina ofan ideal eye forms planar wavefronts at the pupil of the human eye as itleaves the human eye, while the light reflected from the retina of anaberrated eye forms distorted wavefronts at the pupil of the human eyeas it leaves the human eye. The human eye is not ideal and has some formof aberrations such as defocus (which may be myopia (near-sightedness)or hyperopia (far-sightedness)) and astigmatism as well has many otherhigher order optical aberrations.

[0080] The optical elements 59 of the instrument 50 create an image ofthe reflected wavefronts (which represent retroreflection of the imageformed on the retina and exit the pupil of the eye as distortedwavefronts) on a phase compensator 53, which spatially modulates thephase of the image of the reflected wavefronts incident thereon toproduce a compensated image of such reflected wavefronts. The opticalelements 59 recreate this compensated image at the wavefront sensor 55.The wavefront sensor 55 measures the phase aberrations in the wavefrontsincident thereon and operates in a closed-loop fashion with a controller57 to control the phase compensator 53 to compensate for such phaseaberrations to restore the distorted wavefronts to phase-alignedwavefronts, which are directed to the wavefront sensor 55 (for furtherwavefront measurement and compensation if required). Exemplary controlschemes that may be implemented by the controller 57 to control thephase compensator 53 to compensate for such phase aberrations aredescribed by Tyson in “Introduction to Adaptive Optics,” SPIE Press,2000, pgs. 93-109.

[0081] The aberrations of the distorted wavefront measured by thewavefront sensor 55 are characteristic of the aberrations of the eye 1.The wavefront sensor 55 is preferably operably coupled (for example, viaI/O interface 121) to a display device 123 that generates a graphicalrepresentation (such as a wavefront map that depicts the OPD over thepupil, e.g., subapertures, of the wavefront sensor, or a graphicaldisplay of the coefficients of the OPD function) of the aberrations ofthe eye 1 as measured by the wavefront sensor 55.

[0082] As shown in FIG. 3A, the optical elements 59 of the instrument 50preferably include a first polarizing beam splitter 59 and relay lenspair 61/63 that: i) form the image of a wavefront sensing illuminationsource 51 on the retina of the eye 1, which is reflected (and exits thepupil of the eye as distorted wavefronts) and directed back to theinstrument; and ii) direct the reflected wavefronts to a secondpolarizing beam splitter 65 to create an image of the reflectedwavefronts at a phase compensator 53. The phase compensator 53, undercontrol of controller 57, operates to spatially modulate the phase ofthe image of the reflected wavefronts incident thereon to produce acompensated image of such reflected wavefronts that compensate for theaberrations of the eye under examination. The second polarizing beamsplitter 65 and relay lens pair 67/69 recreate this compensated imageproduced by the phase compensator 53 at the wavefront sensor 55 forwavefront sensing.

[0083] Referring now to FIG. 4, there is shown, in schematic form, thefixation target components of an exemplary ophthalmic instrumentaccording to the present invention. As shown, the fixation targetcomponents include an internal fixation target 71 (e.g., a visible imagesource) that cooperates with optical elements 73 to create an image ofthe internal fixation target 71 at the phase compensator 53. The phasecompensator 53, under control of controller 57, operates to spatiallymodulate the phase of the image of the fixation target 71 incidentthereon to compensate for the aberrations of the eye under examinationas measured by the wavefront sensor 55. The optical elements 73 recreatethe phase compensated image of the fixation target 71 produced by thephase compensator 53 at the pupil of the eye 1 under examination. Thisoperation provides the patient with a view of correction (e.g.,compensation) of the aberrations of the eye 1 under examination such thepatient can provide instant feedback as to the accuracy of themeasurement.

[0084] As shown in FIG. 4, the optical elements 73 of the instrument 50preferably include a relay lens pair 77/79 and first polarizing beamsplitter 79 that: i) form an image of the fixation target 71 at thephase compensator 53; and ii) direct the phase compensated image of thefixation target 71 as produced by the phase compensator 53 to a secondpolarizing beam splitter 81. The second polarizing beam splitter 81 andrelay lens pair 83/83 create an image of the phase compensated fixationtarget at the pupil of the eye 1 under examination.

[0085] Referring now to FIG. 5, there is shown, in schematic form, theimaging components of an exemplary ophthalmic instrument according tothe present invention. As shown, the imaging components include animaging illumination source 87 (e.g., halogen flash lamp or xenon flashlamp) that cooperates with optical elements 90 to: i) direct lightproduced from the imaging illumination source 87 onto the pupil of theeye 1, which is reflected and directed back to the instrument; and ii)create an image of these reflections on the phase compensator 53. Thephase compensator 53, under control of controller 57, operates tospatially modulate the phase of such images to compensate for theaberrations of the eye 1 as measured by the wavefront sensor 55. Theoptical elements 90 recreate these phase compensated images produced bythe phase compensator 53 at imaging device 89 (such as a CCD camerabody, 3-CCD camera body, CMOS camera body and/or a photographic filmunit) for capture. This operation provides the user with the capabilityof acquiring high-resolution images of the eye. An image storage andoutput device (not shown) may be operably coupled to the imaging device89 to thereby store the image data captured by the imaging device 89. Inaddition, the image storage and output device may communicate (forexample, over a high speed serial link such as a USB bus) with an imageprocessing and/or display apparatus (not shown) to output the image datastored therein for display, printing and image processing operationsperformed by the image processing and display apparatus.

[0086] As shown in FIG. 5, the optical elements 90 of the instrument 50preferably include a first polarizing beam splitter 91 and relay lenspair 93/95 that: i) direct light produced from the imaging illuminationsource 87 onto the pupil of the eye 1, which is reflected and directedback to the instrument; and ii) direct the reflected wavefronts to asecond polarizing beam splitter 97 to thereby create an image of thereflected wavefronts on a phase compensator 53. In addition, the secondpolarizing beam splitter 97 and relay lens pair 98/99 recreate the phasecompensated image produced by the phase compensator 53 at imaging device89 for capture.

[0087] Referring now to FIG. 6A there is shown, in schematic form, anexemplary embodiment of an ophthalmic instrument 50′ according to thepresent invention, that provides wavefront sensing, an internal fixationtarget and high resolution image capture capabilities.

[0088] Wavefront sensing is provided by a wavefront sensing illuminationsource 51 (e.g., a ring of infrared laser diodes with an characteristicwavelength, for example, of 780 nm) that cooperates with lens 125, beamcombiner 129, first polarizing beam splitter/quarter wave plate 103/105and first relay lens group LG₁ to form an image of the wavefront sensingillumination source 51 on the retina of the eye 1, which is reflected(and exits the pupil of the eye as distorted wavefronts) and directedback to the instrument. The first relay lens group LG₁, first polarizingbeam splitter/quarter wave plate 103/105 and second polarizing beamsplitter/quarter wave plate 109/111 create an image of these distortedwavefronts on phase compensator 53. The phase compensator 53 operates tospatially modulate the phase of the image of the wavefronts incidentthereon. The second polarizing beam splitter/quarter wave plate 109/111,dielectric filter 113, beam folding mirror 117, beam splitter 117 andsecond relay lens group LG₂ recreate the compensated wavefronts producedby the phase compensator 53 at wavefront sensor 55. The dielectricfilter 113 operates to selectively reflect the band of light (e.g.,infrared light with an characteristic wavelength, for example, of 780nm) provided by the wavefront sensing illumination source 51 (and usedfor wavefront sensing) in addition to the band of light provided by theimaging illumination source 97 (and used for image capture), whilepassing the band of light provided by the fixation target 71. Thewavefront sensor 55 measures the phase aberrations in the wavefrontsincident thereon (which are derived from retinal reflections of thewavefront sensing illumination source 51) and operates in a closed-loopfashion with controller 57 to control the phase compensator 53 tospatially modulate the phase of the image of the wavefronts incidentthereon to compensate for such phase aberrations thereon to therebyrestore the distorted wavefronts to phase-aligned wavefronts, which aredirected to the wavefront sensor 55 (for further wavefront measurementand compensation if required).

[0089] The wavefront sensor 55 is preferably operably coupled (forexample, via I/O interface 121) to a display device 123 that generates agraphical representation of the aberrations of the eye 1 as measured bythe wavefront sensor 55. For example, the graphical representation ofthe aberrations of the eye 1 displayed by the display device 123 may bea wavefront map that depicts the OPD over the pupil, e.g., subapertures,of the wavefront sensor, or a graphical display of the coefficients ofthe OPD function as illustrated in FIG. 6C.

[0090] An internal fixation target 71 (e.g., a visible image source)cooperates with a third relay lens group LG₃, dielectric filter 113, andsecond polarizing beam splitter/quarter wave plate 109/111 to create animage of a fixation target 71 at the phase compensator 53. The phasecompensator 53, under control of controller 57, operates to spatiallymodulate the phase of the image of the fixation target 71 to compensatefor the aberrations of the eye under examination as measured by thewavefront sensor 55. The second polarizing beam splitter/quarter waveplate 109/111, first polarizing beam splitter/quarter wave plate103/105, and first lens group LG₁ recreate the phase compensated imageof the fixation target 71 produced by the phase compensator 53 at thepupil of the eye 1 under examination. This operation provides thepatient with a view of correction (e.g., compensation) of theaberrations of the eye 1 under examination such the patient can provideinstant feedback as to the accuracy of the measurement.

[0091] Image capture is provided by an imaging illumination source 87(e.g., halogen or xenon flash lamp) that cooperates with lens 127, beamcombiner 129, first polarizing beam splitter/quarter wave plate 103/105,and first lens group LG₁ to direct light produced from the imagingillumination source 87 onto the pupil of the eye 1, which is reflectedand directed back to the instrument. The first lens group LG₁, firstpolarizing beam splitter/quarter wave plate 103/105, and secondpolarizing beam splitter/quarter wave plate 109/111 create an image ofthese reflections on the phase compensator 53. The phase compensator 53,under control of controller 57, operates to spatially modulate the phaseof such images to compensate for the aberrations of the eye 1 asmeasured by the wavefront sensor 55. The second polarizing beamsplitter/quarter wave plate 109/111, dielectric filter 113, beam foldingmirror 117, beam splitter 117 and fourth relay lens group LG₄ recreatethe compensated image of such reflected wavefronts as produced by thephase compensator 53 at imaging device 89 (such as a CCD camera body,integrating CCD camera body, CMOS camera body and/or a photographic filmunit) for capture. This operation provides the user with the capabilityof acquiring high resolution images of the eye 1.

[0092] As is well known in the art, spectral filters that are tuned tothe wavelength of the wavefront sensing illumination source 51 and/orimaging illumination source 87 may be disposed along the optical pathbetween the beam splitter 117 and the wavefront sensor 55 and imagingdevice 89, respectively, in order to reduce background noise and noisefrom the other illumination sources of the instrument.

[0093] Referring now to FIG. 6B there is shown, in schematic form, anexemplary embodiment of an ophthalmic instrument 50″ according to thepresent invention, that provides wavefront sensing, a fixation targetand high resolution image capture capabilities.

[0094] Wavefront sensing is provided by a wavefront sensing illuminationsource 51 (e.g., a ring of infrared laser diodes with an characteristicwavelength, for example, of 780 nm) that cooperates with lens 125, beamcombiner 129, first polarizing beam splitter/quarter wave plate103′/105′ and first relay lens group LG₁ to form an image of a wavefrontsensing illumination source 51 on the retina of the eye 1, which isreflected (and exits the pupil of the eye as distorted wavefronts) anddirected back to the instrument. The first relay lens group LG₁, firstpolarizing beam splitter/quarter wave plate 103′/105′ and secondpolarizing beam splitter/quarter wave plate 109′/111′ create an image ofthe distorted wavefronts on a phase compensator 53. The phasecompensator 53 operates to spatially modulate the phase of thewavefronts incident thereon. The second polarizing beam splitter/quarterwave plate 109′/111′, dielectric filter 114, beam splitter 117′ andsecond relay lens group LG₂ recreate the image of such compensatedwavefronts at wavefront sensor 55. The dielectric filter 114 operates toselectively reflect the band of light provided by the fixation target71, while passing the band of light (e.g., infrared light with ancharacteristic wavelength, for example, of 780 nm) provided by thewavefront sensing illumination source 51 (and used for wavefrontsensing) in addition to the band of light provided by the imagingillumination source 97 (and used for image capture). The wavefrontsensor 55 measures the phase aberrations in the wavefronts incidentthereon (which are derived from retinal reflections of the wavefrontsensing illumination source 51) and operates in a closed-loop fashionwith a controller 57 to control the phase compensator to spatiallymodulate the phase of the wavefronts incident thereon to compensate forsuch phase aberrations (by warping it's surface to form the complexconjugate of the measured errors) to thereby restore the distortedwavefronts to phase-aligned wavefronts, which are directed to thewavefront sensor 55 (for further wavefront measurement and compensationif required).

[0095] The wavefront sensor 55 is preferably operably coupled (forexample, via I/O interface 121) to a display device 123 that generates agraphical representation of the aberrations of the eye 1 as measured bythe wavefront sensor 55. For example, the graphical representation ofthe aberrations of the eye 1 displayed by the display device 123 may bea wavefront map that depicts the OPD over the pupil, e.g., subapertures,of the wavefront sensor, or a graphical display of the coefficients ofthe OPD function as illustrated in FIG. 6C.

[0096] The fixation target is provided by an internal fixation target 71(e.g., a visible image source) that cooperates with a third relay lensgroup LG₃, dielectric filter 114, and second polarizing beamsplitter/quarter wave plate 109′/111′ to create an image of the internalfixation target 71 at the phase compensator 53. The phase compensator53, under control of controller 57, operates to spatially modulate thephase of the image of the fixation target 71 to compensate for theaberrations of the eye under examination as measured by the wavefrontsensor 55. The second polarizing beam splitter/quarter wave plate109′/111′, first polarizing beam splitter/quarter wave plate 103′/105,′and first lens group LG₁ recreate the phase compensated image of thefixation target 71 produced by the phase compensator 53 at the pupil ofthe eye 1 under examination. This operation provides the patient with aview of correction (e.g., compensation) of the aberrations of the eye 1under examination such the patient can provide instant feedback as tothe accuracy of the measurement.

[0097] Image capture is provided by an imaging illumination source 87(e.g., halogen or xenon flash lamp) that cooperates with lens 127, beamcombiner 129, first polarizing beam splitter/quarter wave plate103′/105′, and first lens group LG₁ to direct light produced from theimaging illumination source 87 onto the pupil of the eye 1, which isreflected and directed back to the instrument pupil. The first lensgroup LG_(1.), first polarizing beam splitter/quarter wave plate103′/105′, and second polarizing beam splitter/quarter wave plate109′/111′ create an image of these reflections on the phase compensator53. The phase compensator 53, under control of controller 57, operatesto spatially modulate the phase of such images to compensate for theaberrations of the eye 1 as measured by the wavefront sensor 55 Thesecond polarizing beam splitter/quarter wave plate 109′/111′, dielectricfilter 114, beam splitter 117′ and fourth relay lens group LG₄ recreatethe compensated image of such reflected wavefronts as produced by thephase compensator 53 at imaging device 89 (such as a CCD camera body,3-CCD camera body, CMOS camera body and/or a photographic film unit) forcapture. This operation provides the user with the capability ofacquiring high resolution images of the eye 1.

[0098] As is well known in the art, spectral filters that are tuned tothe wavelength of the wavefront sensing illumination source 51 and/orimaging illumination source 87 may be disposed along the optical pathbetween the beam splitter 117′ and the wavefront sensor 55 and imagingdevice 89, respectively, in order to reduce background noise and noisefrom the other illumination sources of the instrument.

[0099] In addition, the ophthalmic instrument of the present inventionpreferably includes the following components (which, while not shown inthe Figures in order to simplify the diagram, are assumed provided inthe system described herein):

[0100] Headband and chinrest: the patient is positioned at theinstrument with his forehead against the band and his chin in thechinrest.

[0101] Chinrest adjusting knob: the vertical distance between theforehead band and the chinrest is adjusted with this knob.

[0102] Fixation Target Control knob(s): controls the working distance(and possibly lateral movement in the plane perpendicular to the opticalaxis) of the instrument, and possibly size (i.e., scale)) of theinternal fixation target 71.

[0103] Typically, the working distance of the internal fixation target71 is set to infinity in order to limit the accommodation of the eyeduring wavefront sensing, and/or imaging operations.

[0104] The wavefront sensor 55 of the ophthalmic instrument of thepresent invention preferably comprises a Shack-Hartmann wavefrontsensor, which includes an array of small lenslets disposed in front ofan imaging device (such as a CCD camera body, integrating CCD camerabody or CMOS camera body). The lenslets partition the incident wavefrontinto a large number of smaller wavefronts, each of which is focused to asmall spot on the imaging device. The spatial location of each spot is adirect measure of the local tilt (sometimes referred to as local slopeor local gradient) of the incident wavefront. The Shack-Hartmannwavefront sensor includes signal processing circuitry (for example, adigital signal processor) that samples the output of the imaging deviceand processes the data output there from to track the spatial positionsof these spots to derive the local tilt (e.g., local gradients) of theincident wavefronts. These local gradients are reconstructed to formdata representative of the aberrations of the distorted wavefronts(including defocus, spherical aberration, coma, astigmatism in additionto other higher order aberrations of the distorted wavefronts). Forexample, the local gradients may be reconstructed into an optical pathdifference (OPD) array, which stores a scalar value that represents theoptical path difference at each lenslet. Alternatively, the localgradients may be reconstructed into an OPD function, for example, byminimizing the difference between the derivatives of an analyticalfunction (such as a set of Zernike polynomials, Seidel polynomials,Hermites polynomials, Chebychev polynomials, and Legendre polynomials)and the measured local gradients. A more detailed description ofexemplary Shack-Hartman wavefront sensor configurations are describedbelow. Alternate wavefront sensing techniques are described in detail inGeary, “Introduction to Wavefront Sensors”, SPIE Optical EngineeringPress, 1995, pp. 53-103.

[0105] Alternatively, the wavefront sensor 55 may comprise a Tscherningwavefront analyzer that illuminates the eye with a dot pattern formed bya laser source and dot pattern mask. The reflected dot pattern iscaptured by the imaging device and the image data is analyzed to derivedeviations in the dot pattern from its ideal locations. From theresulting deviations, aberrations in the distorted wavefronts producedfrom the subject eye are mathematically reconstructed. A more detaileddescription of a Tscherning wavefront analyzer is described by Mierdelet al. in “A measuring device for the assessment of monochromaticaberrations of the eye,” Ophthamologe, 1997, Vol. 94, pgs. 441-445, andMrochen et al., “Principles of Tscherning Aberrometry,” J of RefractiveSurgery, Vol. 16, September/October 2000.

[0106] Alternatively, the wavefront sensor 55 may comprise a spatiallyresolved refractometer as described in detail by He et al. in“Measurement of the wave-front aberration of the eye by fastpsychophysical procedure,” J Opt Soc Am A, 1998, Vol. 15, pgs. 2449-2456and in U.S. Pat. No. 5,258,791 and 6,000,800, each incorporated hereinby reference in its entirety.

[0107] Alternatively, the wavefront sensor 55 may comprise any one ofthe improved wavefront sensor configurations described below inconjunction with FIGS. 20A-20D or FIG. 22, or FIGS. 25B, 26, 27A and27B.

[0108] The wavefront sensor 55 measures the aberrations (includingdefocus, spherical aberration, coma, astigmatism in addition to otherhigher order aberrations) of the distorted wavefronts (produced byretinal reflection of light produced by the wavefront sensingillumination source 51). The aberrations measured by the wavefrontsensor 55 represent the aberrations of the subject eye (including highorder aberrations of the eye such as spherical aberration, astigmatismand coma). The wavefront sensor 55 supplies data representative of theseaberrations (such as an OPD array or OPD function) to the controller 57,which controls the phase compensator 53 to restore the distortedwavefronts (which are derived from retinal reflections of the wavefrontsensing illumination source 51) to phase-aligned wavefronts, which aredirected to the wavefront sensor 55 (for further wavefront measurementand compensation if required).

[0109] In another aspect of the present invention, as illustrated inFIG. 7A, the phase compensator 53 embodied within the adaptive opticsubsystem of an ophthalmic instrument (including the ophthalmicinstruments described above) preferably comprises multiple stages (suchas the variable focus lens (VFL) and a deformable mirror as shown) thatcompensate for different parts of the aberrations of the eye 1 asestimated by the wavefront sensor 55. For example, the wavefront sensor55 (or the controller 57) can decompose such aberrations into a defocuscomponent (which represents the defocus of the eye 1) and one or moreadditional components which represent the higher order components (e.g.,spherical aberration, astigmatism and coma) of such aberrations. In thiscase, controller 57 controls the first stage (i.e., the variable focuslens) to compensate for the defocus component of such aberrations, andcontrols the one or more additional stages (i.e., a deformable mirror)to compensate for the remaining higher order components of suchaberrations. A deformable mirror achieves such compensation by warpingits optical surface to form the complex conjugate of such higher ordercomponents as measured by the wavefront sensor 55. In ophthalmicapplications where defocus is the primary component of the aberrationsof the eye, such a configuration improves the dynamic range of the phasecompensation operation performed by the adaptive optic subsystem. Asillustrated in FIG. 7B, the variable focus lens may comprise astationary first lens 1, and a second lens 2 that is moved linearly withrespect to the first lens along the optical axis of the first and secondlens and deformable mirror by an actuator a shown.

[0110] Silicon micro-machined membrane mirrors (which is a class ofdeformable mirrors that are readily available, for example, from OKOTechnologies of Deelft, the Netherlands) are suitable for phasecompensation for many ophthalmic imaging applications. As illustrated inFIG. 8A, such mirrors typically consist of a silicon chip 601 mountedover a printed circuit board substrate 603 by spacers 605. The topsurface 607 of the chip 603 contains a membrane (typically comprisingsilicon nitride) which is coated with a reflective layer (such asaluminum or gold) to form the mirror surface. The printed circuit board603 contains a control electrode structure (as illustrated in FIG. 8B)that operates to deform the shape of the reflective membrane by applyingbias and control voltages to the membrane and the control electrodes609.

[0111] Other classes of deformable mirrors (including segmented mirrors,continuous faceplate mirrors, and edge actuated mirrors) suitable forphase compensation for many ophthalmic applications are described byTyson in “Introduction to Adaptive Optics,” SPIE Press, 2000, pgs.83-91, supra. In addition, classes of liquid crystal devices aresuitable for phase compensation for many ophthalmic applications.

[0112] Proper alignment (and focus) of the optical elements of theophthalmic instrument 50 (including the optical elements of thewavefront sensor 55) is required for optimal operations. In addition,proper alignment of the eye 1 to the ophthalmic instrument 50 (or properalignment of the ophthalmic instrument 50 to the eye 1) is also requiredfor optimal operations.

[0113] Preferably, alignment of the optical elements of ophthalmicinstrument 50 is accomplished by user manipulation of one or morecontrol levers (or joystick(s)) that control forward/backward,side-to-side, and vertical alignment of the optical elements of theinstrument 50. Gross alignment of the instrument 50 is preferablyaccomplished by sliding the base of the instrument 50 in the desireddirection. Focus of the instrument 50 is preferably controlled by one ormore focusing knobs that cooperate with the optical elements of theinstrument to adjust focus of the instrument 1.

[0114] Proper alignment of eye 1 to the instrument 50 may beaccomplished with a headband and chin rest whereby the patient ispositioned at the instrument 50 with his/her forehead against theheadband and his/her chin in the chinrest. One or more adjusting knobsmay be used to adjust the position of the subject eye such that it isproperly aligned with the optical axis of the instrument 50.Alternatively, the position (and orientation) of the instrument 50 maybe changed such that it is properly aligned with the eye 1. This step issuitable for handheld ophthalmic devices. Such alignment is preferablyaccomplished through the use of cross-hairs and an infrared distancedetector embodied within the instrument 50. The cross-hairs are centeredin the field of view of the instrument 50 and viewable to the user suchthat the user can accurately position the cross hairs onto the pupil ofthe eye 1. The infrared distance detector provides visible feedback(i.e., varying frequency flicker lights) or audible feedback (differentpitched beeps) that enables the user to accurately position and orientthe optical axis of the instrument 50 with respect to the eye 1.

[0115]FIG. 9 illustrates exemplary Shack-Hartmann wavefront sensingcomponents that can embodied within the ophthalmic instruments of thepresent invention. As shown in FIG. 9, these components includeforeoptics 301 and a wavefront sensor head 303. The foreoptics 301,which preferably include a beam combiner 304 and collimating lens 305 asshown, operate in conjunction with the optical elements of theinstrument to form an image of the distorted wavefronts (which areformed via reflection of the image of the wavefront sensing illuminationsource 51 on the retina of the eye 1) in the plane of a lenslet array307. The lenslet array 307 partitions the incident wavefront into alarge number of smaller wavefronts and forms corresponding focal spots(e.g., Hartmann spot pattern). A relay lens 309 images the Hartmann spotpattern on an imaging device 311 (such as a CCD camera body, a CMOScamera body, or an integrating CCD camera body). The imaging device 311is operably coupled to an image processor 310 that grabs the image datacaptured by the imaging device 311, processes the grabbed image data totrack spot movement in the Hartmann spot pattern, derives a measure ofthe local tilt of the distorted wavefronts at the lenslets from suchtest spot movements, and possibly stores such image data in persistentstorage. In addition, the image processor 310 generates data (such as anOPD array or OPD function) representative of the aberrations of thedistorted wavefronts (including defocus, spherical aberration, coma,astigmatism in addition to other higher order aberrations of thedistorted wavefronts) from such measures. In adaptive opticapplications, such as the ophthalmic instruments described above, suchdata is provided to a controller which controls a phase-compensatingoptical element(s) to compensate for such phase aberrations to restorethe distorted wavefronts to phase-aligned wavefronts.

[0116]FIG. 10A illustrates an exemplary embodiment of the components ofimaging device 311 of FIG. 9, including a CCD array 811 ofphotodetectors that detect the intensity of incident light thereon andgenerate an electrical signal in response thereto, timing and controlcircuitry 813 that supply timing signals to the CCD array 811 to: readout the electrical signals generated by the elements therein, store thesignals in buffer 815, output the signals stored in buffer 815 to signalprocessing circuitry 817 that condition such signals foranalogue-to-digital conversion circuitry 819, and store digital datawords (pixel data words) derived from such signals in digital buffer andoutput circuitry 821 for output to image processing. Alternatively, aCMOS array or integrating CCD array may be substituted for the CCD array811.

[0117]FIG. 10B illustrates an exemplary embodiment of the components ofimage processor 310 of FIG. 9, including a memory controller 853 thatprovides access to memory 855 for interface 851, digital signalprocessor 857. Interface 851 inputs pixel data words from the imagingdevice 311 and stores such pixel data words in memory 855 via memorycontroller 853. The digital signal processor 857 accesses the pixel datastored in memory 855 and processes such data in accordance with asequence of programmed instructions. The memory controller 853 alsocommunicates with an I/O interface to thereby display data on a displaydevice (such as a TFT LCD device).

[0118] As illustrated in FIG. 11, the Hartmann spot pattern is formed atapproximately a lenslet focal length f_(L) behind the lenslet array 307.For a number of reasons, it is desirable to use relay lens 309 to relaythe Hartmann spot pattern onto the imaging device 311. First, thisallows the matching of the scale of the Hartmann spot pattern to thepixel size of the imaging device 311. Second, it simplifies theimplementation of interchangeable lenslets (of varying focal lengthand/or aperture size). Finally, it allows the wavefront sensor head 303to gather a much wider range of data on the optical system under testand, as a result, make measurements of greater accuracy. Preferably, therelay lens 309 operates in a telecentric mode to minimize thepossibility of magnification errors that lead to wavefront estimationerrors.

[0119] The lenslet array 307 may be of the type manufactured and sold byAdaptive Optics Inc, of Cambridge, Mass., assignee of the presentinvention, in that it comprises a precision array of refractivemicrolenses formed continuously on a monolithic substrate. The array ofmicrolenses are preferably compression molded of polymethymethacrylate(PMMA) plastic, and positioned in the substrate with full edge-to-edgelenslet contact to maximize the density of lens area to total surfacearea (referred to as “fill factor”). The fill factor determines how muchof the scaled full aperture system pupil (the subject eye underexamination) is captured by the array. The commercial lenslet arraysmanufactured and sold by assignee of the present invention have fillfactors exceeding 98 percent.

[0120] As described above, the Shack-Hartmann wavefront sensing head 303derives local wavefront tilt at a given lenslet from spot motion for thegiven lenslet. Such derivation inherently depends upon the particulargeometry of the sensor head and its optics (including the distancebetween the pupil image plane and the spot image plane, the radius ofthe pupil of the lenslet, and possibly the refractive index of thelenslet) and requires a geometric reference of the nominal null (i.e.,spot location corresponding to incident of a planar wavefront on a givenlenslet). A more detailed description of the derivation for relatingspot motion to local wavefront tilt is described in detail by Geary in“Introduction to Wavefront Sensors”, SPIE Optical Engineering Press,1995, pp.14-20. Because the distance between the pupil image plane andthe spot image plane and the radius of the pupil of the lenslet (whichis set by the size of the individual lenslet elements) are bothquantities that are determined at the time of manufacture, this basicparameters need not be re-measured each time the system is used.

[0121] There are, however, a number of parameters related to aparticular wavefront measurement that must be determined before thatparticular wavefront measurement can be made. These parameters includethe geometric reference of nominal null and, possibly, the position andshape of the system pupil (e.g., the pupil of the eye under test) in thelocal coordinate system of the wavefront sensing head 303. The shape ofthe system pupil is primarily used for the calculation of polynomialdecompositions of the wavefront. For example, Zernike and Seidelpolynomial decompositions are derived from a circular pupil, whereasMonomials, Hermites, Chebychev, and Legendre polynomial decompositionsare derived from a rectangular pupil. However, selection of the pupilshape has no direct effect on the wavefront measurement itself. In caseswhere there is no well defined pupil, any convenient pupil may beselected.

[0122] Geometric Reference of Nominal Null

[0123] The Shack-Hartmann wavefront sensor (which is preferably embodiedwithin the ophthalmic instruments of the present invention) may utilizeone of a number of different approaches in achieving the geometricreference to nominal null.

[0124] A first approach achieves this geometric reference by a referenceplane wave (generated by a laser source and suitable collimating opticalelements) that is recreated at the plane of the lenslet array 307. TheHartmann spot pattern, which is formed at approximately a lenslet focallength f_(L) behind the lenslet array 307, is captured by imaging device311. The image processor 310 is controlled to grab the image captured bythe imaging device 311 and process this image to derive reference spotlocations (based upon the centroid of the spots therein). Duringwavefront sensing operations, deviation of Hartmann spot location (withrespect to the recorded reference spot location) is measured to derivean estimate of the phase aberration in the wavefront sampled by thecorresponding lenslet. This first approach is costly because the flatwave signal source and collimating optics must be of high opticalquality.

[0125] A second approach achieves this geometric reference (without thecosts of a high quality flat wave signal source and collimating optics)by providing a substantially-opaque element at the center of eachlenslet of the lenslet array 307′ as illustrated in FIG. 12. The lensletarray 307′ is illuminated with a reference beam, which is preferablyproduced by reference source 315 and directed to the lenslet array 307′by beam combiner 304 and collimating lens 305 as shown in FIG. 9.Concurrently therewith, an imaging device is positioned conjugate thefiducial point image plane—this is the plane whereby the fiducial pointsappear at the imaging device as an array of fiducial reference spotsexactly co-aligned with the centers of the lenslets. An image processoris controlled to grab the image captured by the imaging device andprocess this image to identify the locations of the fiducial referencespots (based upon the centroid of the fiducial reference spots).Deviation of Hartmann spot location (with respect to the recordedfiducial reference spot location) is measured during wavefront sensingoperations to measure the phase aberration in the wavefront sampled bythe corresponding lenslet. Advantageously, the optical components thatimage the reference beam onto the lenslet array 307′ may be of loweroptical quality and costs than the optical components required toprovide the flat reference wave as discussed above. In addition, thesecond approach is far more stable than the first approach.

[0126] This second approach may be accomplished with the wavefrontsensing head 303′ of FIGS. 13A and 13B wherein relay lens 309′ and theimaging device 311′ are mounted on a linear actuator that has sufficienttravel to allow the imaging device 311′ to image all planes from theplane substantially near the lenslet array 307′ itself, back to thefocal plane of the longest focal length lenslet array. Thisconfiguration is described in detail in U.S. Pat. No. 5,629,765,commonly assigned to the assignee of the present invention, and hereinincorporated by reference in its entirety. In this configuration,location of the fiducial reference spots in the fiducial reference spotpattern is accomplished by moving the relay lens 309′ and imaging device311′ as illustrated in FIG. 13A such that the imaging device 311′ isplaced conjugate the fiducial point image plane. A reference beamilluminates the lenslet array 307′, and the image processor 310 iscontrolled to grab one or more images of the fiducial point image planeas captured by the imaging device 311, and process such image(s) toidentify the locations of the fiducial reference spots for each givenlenslet.

[0127] The phase aberration in the distorted wavefront sampled by agiven lenslet is approximated by determining the location of theHartmann spot produced by the given lenslet relative to the location ofthe fiducial reference spot corresponding to the given lenslet. Asillustrated in FIG. 13B, this measurement is accomplished by moving therelay lens 309′ and imaging device 311′ such that the imaging device311′ is placed at one or more positions substantially near the spotimage plane—this is plane whereby the focal plane of the lenslet array307′ as illustrated in FIG. 11 is imaged onto the imaging device 311.The distorted wavefront is recreated at the plane of the lenslet array307′, and the image processor 310 is controlled to grab one or moreimages substantially near the spot image plane as captured by theimaging device 311, and process such image(s) to: (i) identify thelocation of the Hartmann spot corresponding to the lenslets, and (ii)compute the relative difference between this Hartmann spot location andthe corresponding location of the fiducial reference spot for thelenslets.

[0128] A third approach achieves this geometric reference (without thecosts of a high quality flat wave signal source and collimating opticsof the first approach and without the costs of adding fiducial points tothe lenslets of the lenslet array as described above in the secondapproach) using sophisticated image processing techniques on multipleimages of the pupil image plane. FIG. 15 is a flow chart illustrating anexemplary embodiment of such image processing techniques.

[0129] In step 1501, the lenslet array 307 is illuminated by a referencesource (for example, light produced by reference source 315 may bedirected to the lenslet array 307 by beam combiner 304 and collimatinglens 305 as shown in FIG. 9). Preferably, the apparent distance of thereference source to the lenslet array 305 is (or is near) infinity tominimize magnification changes (due to change in distance) in the firstand second images of the pupil image plane as described below.Concurrently therewith, an imaging device is positioned such that itcaptures a first image of pupil image plane that shows the edges of thelenslets as a dark grid, and an image processor is controlled to grabthis first image captured by the imaging device.

[0130] In step 1503, the lenslet array 307 is illuminated by thereference source; and, concurrently therewith, an imaging device ispositioned such that it captures a second image of pupil image planethat shows the edges of the lenslets as a bright grid, and the imageprocessor is controlled to grab this second image captured by theimaging device.

[0131] In step 1505, the image processor generates a third compositeimage representing the grid (with the average signal of the first andsecond images removed) by subtracting the first and second images.

[0132] In step 1507, the image processor calculates the real andimaginary parts of a two-dimensional Fourier transform of the thirdcomposite image generated in step 1505. The two-dimensional Fouriertransform can generally be represented as follows:${F( {k,l} )} = {\frac{1}{N\quad M}*{\sum\limits_{x = 0}^{N - 1}{\sum\limits_{y = 0}^{M - 1}{{f( {x,y} )}^{- {{2\Pi}{({\frac{k\quad x}{N} + \frac{l\quad y}{M}})}}}}}}}$

[0133] where N represents the number of pixels in each row (i.e., xdirection) of the image;

[0134] M represents the number of pixels in a column (i.e., y direction)of the image; f(x,y) represents the intensity value at a pixel (x,y) inthe image; and

[0135] the exponential term is the basis function corresponding to eachpoint F(k,l) in Fourier space.

[0136] This transform can be calculated as a double sum at each imagepoint as follows:${F( {k,l} )} = {\frac{1}{M}*{\sum\limits_{y = 0}^{M - 1}{{P( {k,y} )}^{{- {2\Pi}}\frac{ly}{M}}}}}$w  h  e  r  e${P( {k,y} )} = {\frac{1}{N}*{\sum\limits_{x = 0}^{N - 1}{{f( {x,y} )}^{{- {2\Pi}}\frac{kx}{N}}}}}$

[0137] The two-dimensional Fourier transform F(k,l) can be decomposedinto a real part (R(k,l)) and an imaginary part (I(k,l)) as follows:

F(k,l)=R(k,l)+iI(k,l)

[0138] In step 1508, the image processor calculates the magnitude andphase components of the two-dimensional dimensional Fourier transformcalculated in step 1507. The magnitude and phase components of thetwo-dimensional Fourier transform F(k,l) can be calculated as follows:

Magnitude |F(k,l)|=(R(k,l)² +I(k,l)²)^(1/2)${P\quad h\quad a\quad s\quad e\quad {\Phi ( {k,l} )}} = {\arctan ( \frac{I( {k,1} )}{R( {k,1} )} )}$

[0139] In step 1509, the image processor determines the period of thegrid based upon the magnitude component of the two-dimensionaldimensional Fourier transform calculated in step 1508, and determinesthe location of the grid based upon the phase component of thetwo-dimensional dimensional Fourier transform calculated in step 1508.This step is preferably accomplished as follows. First, a region aroundthe expected first-harmonic peak (based upon the approximate spacing ofthe grid) in the magnitude component calculated in step 1508 isidentified. Second, a function (such as a parabolic function) is fit tothis region, and a maximum of this function is identified. Third, theperiod of a grid in both x and y direction that corresponds to thismaximum is identified. The maximum magnitude component of the Fouriertransform corresponds to a spatial frequency that is inverselyproportional to the period of the grid in both x and y direction.Finally, the phase component of the Fourier transform calculated in step1508 that corresponds to this maximum is identified, and the location ofthe grid is calculated based upon the identified phase component.Conceptually, the identified phase component provides translation inboth x direction and y direction from the origin of the coordinatesystem of the image device to the origin of the grid.

[0140] Finally, in step 1511, the image processor generates a regulargrid of lenslet center locations in the coordinate system of the imagingdevice based upon the location and period of the grid determined in step1509. Conceptually, the lenslet center locations are offset a half cyclein both the x direction and y direction from the grid. The half cycleoffset can be readily determined from period of the grid (which isinversely proportional to the maximum magnitude component of the Fouriertransform).

[0141] Such lenslet center locations are analogous to the locations ofspots of the reference spot pattern in the first approach and thefiducial point spot pattern in the second approach, thus providing thegeometric reference to nominal null (i.e., spot location correspondingto incident of a planar wavefront on a given lenslet). Deviation ofHartmann spot location (with respect to the corresponding lenslet centerlocation) is measured during wavefront sensing operations to measure thelocal tilt in the wavefront sampled by the corresponding lenslet. Thisapproach is advantageous because it avoids the costs of a high qualityflat wave signal source and collimating optics of the first approach andthe costs of adding fiducial points to the lenslets of the lenslet arrayas described above in the second approach.

[0142] This third approach may be accomplished with the wavefrontsensing head 303″ of FIGS. 14A and 14B wherein the relay lens 309 andthe imaging device 311 are mounted on a linear actuator, whichpreferably has sufficient travel to allow the imaging device 311 toimage all planes from the plane substantially near the lenslet array 307itself, back to the focal plane of the lenslet array. In thisconfiguration, the imaging device 311 may capture the first image instep 1501 by moving the relay lens 309 and image device 311 to aposition whereby the imaging device 311 captures the pupil image planeslightly outside of best focus (i.e., the plane of the lenslet array 307and the plane of the relay lens 309 is offset by D+π₁). In addition, theimaging device 311 may capture the second image in step 1503 by movingthe relay lens 309 and image device 311 to a position whereby theimaging device 311 captures the pupil image plane slightly inside ofbest (i.e., the plane of the lenslet array 307 and the plane of therelay lens 309 is offset by D−π₂). The image processor 310 performs theoperations of steps 1505 to 1511 to generate the lenslet centerlocations in the coordinate system of imaging device 311.

[0143] Position of the System Pupil (e.g. the Pupil of the Eye UnderTest) in the Local Coordinate System of the Wavefront Sensing Head

[0144] As described above, the wavefront measurement operationsperformed by the Shack-Hartmann wavefront sensor requires calibration ofthe position of the system pupil (e.g., the pupil of the eye under test)in the local coordinate system of the wavefront sensing head. FIG. 16illustrates an exemplary local coordinate system for use by thewavefront sensing head of FIG. 9, which includes pixel coordinates (u,v)of the imaging device 311 and a distance f along the optical axis of thesystem. Such calibration may be fixed by design. However, this mappingis critical to successful estimation of the wavefront (and the operationof the closed-loop phase compensation system), and any errors thereinare directly related to the absolute wavefront measurement error. Thus,it is preferable that the Shack-Hartmann wavefront sensor employ amechanism that can dynamically (and automatically without humanintervention) perform such calibration.

[0145]FIG. 17 is a flow chart illustrating an exemplary image processingtechnique that automatically locates the position of the system pupil(e.g., the pupil of the eye under test) in the local coordinate systemof the wavefront sensor. This technique can be used dynamically toperform such calibration as frequently as required to ensure theopto-mechanical stability of the system.

[0146] In step 1701, the system pupil (e.g., the pupil of the eye undertest) is imaged at the plane of the lenslet array 307 (for example bythe optical train of the ophthalmic instrument, beam combiner 304 andcollimating lens 305 of the wavefront sensor); and, concurrentlytherewith, an imaging device captures an image of pupil image plane atbest focus.

[0147] In step 1703, an image processor processes the image of the pupilimage plane grabbed in step 1701 to locate the centroid of the image. Tocalculate the centroid of the image in the x-direction, weights areassigned to each column of pixels in the image and the measuredintensity for each pixel in the image is multiplied by the weightcorresponding to the column of the given pixel and summed together. Ifthe weights vary linearly with the distance of the column from thecenter of the image, this sum will be a measure of the x-position of thelight distribution. The sum needs to be normalized by dividing by thesum of the unweighted intensities. To calculate the centroid of thelight distribution in the y-direction, weights are assigned to each rowof pixels in image and the measured intensity for each pixel in theimage is multiplied by the weight corresponding to the row of the givenpixel and summed together. If the weights vary linearly with thedistance of the column from the center of the image, this sum will be ameasure of the y-position of the light distribution. The sum needs to benormalized by dividing by the sum of the unweighted intensities. Suchcentroid calculation may be represented mathematically as follows:$x_{c} = \frac{\sum\limits_{i}{\sum\limits_{j}{w_{j}*I_{ij}}}}{\sum\limits_{i}{\sum\limits_{j}I_{ij}}}$$y_{c} = \frac{\sum\limits_{i}{\sum\limits_{j}{w_{i}*I_{ij}}}}{\sum\limits_{i}{\sum\limits_{j}I_{ij}}}$

[0148] where i and j identify the rows and columns, respectively, of theimage;

[0149] w_(i) and w_(j) are the weights assigned to given rows andcolumns, respectively, of the image; and I_(ij) is the intensity of agiven pixel in row i and column j of the image.

[0150] In step 1705, the image processor processes the image of thepupil image plane grabbed in step 1701 to determine, for a plurality ofslices (e.g., radial cuts) from the centroid (calculated in step 1703)to the periphery of the image, the gradient of the intensity along eachslice, and the pixel location of maximum of the intensity gradient alongeach slice. FIG. 18 provides a graphical illustration of exemplaryslices (RC1 . . . RC8) from a centroid C to the periphery of an image ofthe pupil image plane (e.g., in the u,v pixel space).

[0151] In step 1707, the image processor fits a predetermined shape(such as a circle, ellipse or rectangle) to the pixel locations of themaximums of the intensity gradient along the slices as calculated instep 1705. This best-fit shape (e.g., pixel coordinates of the centerand radius in the case of a best-fit circle) provides the location ofthe system pupil (e.g., the pupil of the eye under test) in the pixelcoordinate system of the imaging device, form which can be derived thelocation of the system pupil (e.g., the pupil of the eye under test) inthe local coordinate system of the wavefront sensing head. For example,for the local coordinate system illustrated in FIG. 16, such pixelcoordinates and the distance along the optical path from the lensletarray 307 to the imaging device 311 provides the (u,v,f) coordinates ofthe system pupil (e.g., the pupil of the eye under test).

[0152] The calibration operations of FIG. 17 may be accomplished withthe wavefront sensing head 303″ of FIGS. 14A and 14B wherein the relaylens 309 and the imaging device 311 are mounted on a linear actuator,which preferably has sufficient travel to allow the imaging device 311to image all planes from the plane substantially near the lenslet array307 itself, back to the focal plane of the lenslet array. In thisconfiguration, the imaging device 311 may capture the image of pupilimage plane at best focus in step 1701 by moving the relay lens 309 andimage device 311 to a position whereby the imaging device 311 capturesthe pupil image plane at best focus (i.e., the plane of the lensletarray 307 and the plane of the relay lens 309 is offset by D). The imageprocessor 310 performs the operations of steps 1703 to 1707 to locatethe position of the system pupil (e.g., the pupil of the eye under test)in the local coordinate system of the wavefront sensing head.

[0153] Dynamic Identification of Sub-Arrays (Pixel Areas) of the ImagingDevice that Avoid Dot-Crossover

[0154] In addition, it is preferable that the Shack-Hartmann wavefrontsensor employ a mechanism that addresses dot crossover problem, whichoccurs when there is a predefined region of the Hartmann spot imagingplane that is used to determine location of the Hartmann spot for agiven lenslet. If the spot moved outside that predefined region (i.e.,dot crossover occurs), the dynamic range of the sensor is exceeded,resulting in an erroneous wavefront measurement.

[0155] In order to address the dot crossover problem, the Shack-Hartmannwavefront sensor can employ a mechanism that dynamically identifies thesub-arrays (pixel areas) of the Hartmann spot imaging device (e.g., theimaging device that will be used for the determination of Hartmann spotpositions) that avoids dot crossover for a particular wavefrontmeasurement. A detailed description of an illustrative procedure thatprovides such a dynamic mechanism is described below with respect toFIG. 19.

[0156] In step 1901, the system pupil image is recreated at the lensletarray 307 and the Hartmann spot imaging device is positioned such thatit captures an image of the Hartmann spot pattern (at best spot focus)as formed at the focal plane of the lenslet array; and the sub-regions(i.e., pixel areas) of the imaging device 311, which are denoted “testsubapertures” for the sake of description, that are to be used for thedetermination of Hartmann spot locations are defined. In this step, thesystem preferably grabs an image of the Hartmann spot pattern andlocates the position of all of the “useable” Hartmann spots in thisimage. Preferably, a predetermined criterion (for example, based uponintensity values of pixels covered by a given Hartmann spot) is used todistinguish between “useable” and “unuseable” Hartmann spots and tofilter out such “unuseable” Hartman spots. Sub-regions of the imagingdevice 311 around each useable Hartmann spot are defined and stored in alist of test subapertures. The sizes of these sub-regions are made aslarge as possible without overlapping. In addition, the systemdetermines if a reasonable number of “useable” Hartmann spots have beenfound based upon the known spacing of the lenslet array 307 and the sizeof the Hartmann spot imaging device. If an unreasonably low number of“useable” Hartmann spots have been found, preferably an error isreported to the user.

[0157] In step 1903, each test subaperture in the list of testsubapertures is matched to a corresponding lenslet (i.e., the particularlenslet that produced the spot from which the subaperture is derived).This matching process preferably is accomplished by capturing, grabbingand processing one or more additional images of the Hartmann spotpattern that are taken slightly inside (or outside) best focus. In suchimage(s), the location of the spot in each subaperture differs from thatfound in the image at best spot focus. This difference is due to anydeviation of the direction of propagation from the optical axis of thelenslet. The positions measured in the images may be used to project aray from a given spot back to the plane of the lenslet array 307 (i.e.,the ray passing through the spot positions as illustrated in FIG. 19B)to generate a list of crossing locations at this plane for each testsubaperture.

[0158] It is important to realize that the test subapertures (defined instep 1901) are wholly separate from the sub-regions of the imagingdevice that will be used for the measurement of the reference source(which are defined in step 1905). It is this use of separate lists ofsubapertures and subsequent matching process (step 1909) that allows thewave front sensor 303 to effectively resolve potential dot crossoverproblems and thus achieve very large dynamic range that includes highlyaberrated eyes.

[0159] In step 1905, a reference source illuminates the lenslet array307 and the Hartmann spot imaging device is positioned such that itcaptures an image of the spot pattern (at best spot focus) formed at thefocal plane of the lenslet array; and the sub-regions (i.e., pixelareas) of the imaging device, which are denoted “reference subapertures”for the sake of description, that are to be used for the determinationof such spot locations are defined. The reference source may be directedto the lenslet array 307 by beam combiner 304 and collimating lens 305as shown in FIG. 9. In this step, the system preferably grabs an imageof the spot pattern and locates the position of all of the “useable”spots in this image. Preferably, a predetermined criterion (for example,based upon intensity values of pixels covered by a given spot) is usedto distinguish between “useable” and “unuseable” spots and to filter outsuch “unuseable” spots. Sub-regions of the imaging device around eachuseable spot are defined and stored in a list of reference subapertures.The sizes of these sub-regions are made as large as possible withoutoverlapping. In addition, the system determines if a reasonable numberof “useable” spots have been found based upon the known spacing of thelenslet array 307 and the size of the imaging device. If an unreasonablylow number of “useable” spots have been found, preferably an error isreported to the user.

[0160] In step 1907, each reference subaperture in the list of referencesubapertures is matched to a corresponding lenslet (i.e., the particularlenslet that produced the spot from which the subaperture is derived).This matching process preferably is accomplished by capturing, grabbingand processing one or more additional images of this spot pattern thatare taken slightly inside (or outside) best focus. In such image(s), thelocation of the spot in each subaperture differs from that found in theimage at best spot focus. This difference is due to any deviation of thedirection of propagation from the optical axis of the lenslet. Thepositions measured in the images may be used to project a ray from agiven spot back to the plane of the lenslet array 307 (i.e., the raypassing through the spot positions as illustrated in FIG. 19B) togenerate a list of crossing locations at this plane for each referencesubaperture.

[0161] In step 1909, the system then processes the lists of crossinglocations and the associated aperture lists to find unique referenceaperture/test aperture pairs whose crossing points coincide withinprescribed tolerances to a lenslet center of the lenslet array (whichmay be derived from the reference spot locations as described above, orfrom the fiducial point locations as described above with respect toFIGS. 12, 13A, 13B, and 14, or from the image processing techniquesdescribed above with respect to FIGS. 15 and 16). The system thenverifies that the crossing points for these unique referenceaperture/test aperture pairs correspond to a single lenslet in thelenslet array. In the event that the crossing points for a particularreference aperture/test aperture pair correspond to different lensletsin the lenslet array, that particular reference aperture/test aperturepair is removed from the list. The ultimate result of the matchingprocess of step 1909 is a list of lenslets (or reference spot locationsor fiducial point locations or lenslet centers) each uniquely associatedwith a given reference subaperture and test subaperture.

[0162] The list of lenslets (or lenslet reference spot locations orlenslet fiducial point locations or lenslet centers) produced by thematching process of step 1909 is used during wavefront sensingoperations to provide the geometric reference of nominal null (i.e.,reference spot position) for the corresponding subapertures. Inaddition, the subapertures of the Hartmann spot image imaging devicethat are used during such wavefront sensing operations is limited to thesubapertures corresponding to the lenslets (or lenslet reference spotlocations or lenslet fiducial point locations or lenslet centers)produced by the matching process of step 1909, thereby effectivelyresolving the dot cross over problem.

[0163] It is important to realize that the reference subapertures(defined in step 1901 are wholly separate from the test subapertures(defined in step 1905). It is this use of separate lists of subaperturesand the subsequent matching process (steps 1903, 1907 and 1909) thatallows the wavefront sensor to effectively resolve potential dotcrossover problems and thus achieve very large dynamic range thatincludes the wavefront sensing of highly aberrated eyes. Moreover, thisprocess may be repeated during measurement to verify calibration (orpossibly recalibrate) of the wavefront sensor.

[0164] The operations of FIG. 19 require the capture of multiple imagesof different planes between the lenslet array itself and the focal planeof the lenslet array. Such operations may be accomplished with thewavefront sensing head 303″ of FIGS. 14A and 14B (or similarly with thewavefront sensing head 303′ of FIGS. 13A and 13B) wherein the relay lens309 and the imaging device 311 are mounted on a linear actuator, whichpreferably has sufficient travel to allow the imaging device 311 toimage all planes from the plane substantially near the lenslet array 307itself, back to the focal plane of the lenslet array. In thisconfiguration, the imaging device 311 captures the images of the testspot pattern/reference spot pattern at best focus in steps 1901/1905 bymoving the relay lens 309 and image device 311 to a position whereby theimaging device 311 captures the spot image plane at best focus (i.e.,the plane of the lenslet array 307 and the plane of the relay lens 309is offset by D+f_(L)). And, the imaging device 311 captures the imagesof the test spot pattern/reference spot pattern slightly inside (oroutside best focus) in steps 1903/1907 by moving the relay lens 309 andimage device 311 to a position whereby the imaging device 311 capturesthe spot image plane slightly inside (or outside) best focus (i.e., theplane of the lenslet array 307 and the plane of the relay lens 309 isoffset by D+f_(L)±τ).

[0165] Note that the configuration of wavefront sensing head 303″ ofFIGS. 14A and 14B (and similarly the configuration of the wavefrontsensing head 303′ of FIGS. 13A and 13B) achieve the capture of multipleimages required for full wavefront measurement by moving a singleimaging device. Such configurations lead to unavoidable delay betweensuch captures. This delay may be problematic in some ophthalmicapplications. More specifically, unless the human eye is immobilized, itis constantly moving, both voluntarily and involuntarily. While suchmotion can be frozen by using a short exposure time to capture theHartmann spots, when the pupil of the eye moves from exposure toexposure, it becomes difficult to correctly associate the local tiltmeasures made by the wavefront sensor with the proper location on thepupil of the eye. The situation is further complicated by the fact thatthe pupil size is also changing as the eye adjusts to light level orother stimuli. These effects can be significant if there is a long delaybetween exposures (which leads to significant eye movement) and resultin unacceptable wavefront measurement errors.

[0166] In addition, unless the accommodation of the human eye isparalyzed using drugs (such as Cycloplegics), the eye is capable ofaccommodation. During wavefront sensing, such accommodation may lead tomeasurement errors and misdiagnosis. Accomodation is typicallycontrolled by directing the patient to focus on an internal fixationtarget, whose working distance is set to infinity to limit suchaccommodation. However, when measuring the aberrations of the eyes ofchildren at an early age (for example, early age screening for visionimpairments, such as ambiopia that are more easily correctable at suchearly ages), controlling accommodation at such an early age is verydifficult because the child cannot understand the directions of theoperator to focus on the internal fixation target. In suchcircumstances, drugs that paralyze accommodation (commonly calledCycloplegics) are typically used to measure the aberrations of the eye.Such drugs typically require a long time to take effect (30-60 minutes).In the event that the required time period for inducement of paralysisis not satisfied, prior art ophthalmic instruments are error prone inmeasuring the aberration of the eye and thus susceptible tomisdiagnosis. In addition, such drugs typically require a long wear off(up to 24 hours), which leads to patient discomfort over this prolongedperiod.

[0167] Real-Time Hartmann Wavefront Sensing

[0168] In another aspect of the present invention, improved Hartmannwavefront sensors (and ophthalmic instruments employing the same) havebeen developed that address the challenges presented by eye movementand/or accommodation.

[0169]FIG. 20A illustrates an improved Shack-Hartmann wavefront sensinghead 303′″ that utilizes fiducial points of the lenslet array to providea geometric reference of nominal null and avoids delays associated withcapture of the required multiple images. The improved wavefront sensinghead 303′″ includes a relay lens 351, beam splitter 353 and multipleimaging devices that cooperate to capture images of the fiducial pointimage plane and the Hartmann spot imaging plane. In this configuration,the beam splitter 353 splits the light beams produced by the lensletarray 307′ into multiple arms (preferably of equal intensity) which areprojected onto the multiple imaging devices. For example, as shown, beamsplitter 353 splits the light beams produced by the lenslet array 307′into two arms (preferably of equal intensity) which are projected ontoimaging devices 311-A and 311-B. The beam splitter 353 may comprise oneor more beam splitting elements, such as a cube-type beam splitter,plate-type beam splitter, or dichroic prism assembly such as thosereadily available from: Richter Enterprises of Livingston, TX, describedin detail at http://www.techexpo.com/WWW/richter/; DuncanTech of Auburn,Calif., described in detail at http://www.duncantech.com/; and EdmudOptics of Barrington, N.J., described in detail athttp://www.edmundoptics.com/.

[0170] In the illustrative embodiment of FIG. 20A, imaging device 311-Ais positioned at the fiducial point image plane and is used to capturethe array of fiducial reference spots. A reference beam illuminates thelenslet array 307′, and image processor 310 is controlled to grab one ormore images of the fiducial point image plane as captured by the imagingdevice 311-A, and process such image(s) to identify the locations of thefiducial reference spots for each given lenslet. In addition, at leastone other imaging device 311-B is placed at a position substantiallynear the Hartmann spot image plane and is used to capture images of theHartmann spot pattern. During wavefront sensing operations, thedistorted wavefront is recreated at the plane of the lenslet array 307′,and the image processor 310 is controlled to grab one or more imagessubstantially near the spot image plane as captured by the imagingdevice 311-B, and process such image(s) to: (i) identify the location ofthe Hartmann spot corresponding to the lenslets, and (ii) compute therelative difference between this Hartmann spot location and thecorresponding location of the fiducial reference spot for the lenslets.

[0171] Preferably, the illumination of the reference beam and subsequentimage grabbing and processing operations that derive the locations ofthe fiducial reference spots for each given lenslet are performed as acalibration operation prior to wavefront sensing measurements for aneye. Moreover, such operations can be used dynamically (in a mannerconcurrent with or interposed with such wavefront sensing measurements)to perform such calibration as frequently as required to ensure theaccuracy of the system. Such dynamic operations (which may be concurrentwith or interposed with wavefront sensing measurements) enable accuratereal-time wavefront measurement while the eye moves and/or accommodates.

[0172]FIG. 20B illustrates an improved Shack-Hartmann wavefront sensinghead 303″″ that utilizes the image processing techniques on multipleimages of the pupil image plane to derive the geometric reference tonominal null (as described above with respect to FIG. 15) and avoiddelays associated with the capture of the required multiple images. Theimproved wavefront sensing head 303″″ includes a relay lens 351, beamsplitter 353 and multiple imaging devices that cooperate to captureimages of the pupil image plane and the Hartmann spot imaging plane. Inthis configuration, the beam splitter 353 splits the light beamsproduced by the lenslet array 307 into multiple arms (preferably ofequal intensity) which are projected onto the multiple imaging devices.For example, as shown, beam splitter 353 splits the light beams producedby the lenslet array 307 into two arms (preferably of equal intensity)which are projected onto imaging devices 311-A and 311-B. Hereto, thebeam splitter 353 may comprise one or more beam splitting elements, suchas a cube-type beam splitter, plate-type beam splitter, or dichroicprism assembly as described above.

[0173] In the illustrative embodiment of FIG. 20B, imaging device 311-Ais positioned substantially near the pupil image plane and is mounted ona linear actuator that enables the image device 311-A to capture thefirst and second images in steps 1501 and 1503, respectively. Areference beam illuminates the lenslet array 307, and image processor310 is controlled to grab the first and second images captured by theimaging device 311-A, and process such image(s) as described above insteps 1505-1511 to identify the lenslet center locations in the localcoordinate system of the imaging device 311-A. In addition, at least oneother imaging device 311-B is placed at a position substantially nearthe Hartmann spot image plane and is used to capture images of theHartmann spot pattern. During wavefront sensing operations, thedistorted wavefront is recreated at the plane of the lenslet array 307,and the image processor 310 is controlled to grab one or more imagessubstantially near the spot image plane as captured by the imagingdevice 311-B, and process such image(s) to: (i) identify the location ofthe Hartmann spot corresponding to the lenslets, and (ii) compute therelative difference between this Hartmann spot location and thecorresponding location of the lenslet center locations.

[0174] In an alternative embodiment of the wavefront sensor head of FIG.20B, multiple image devices may be used to capture the image of thepupil image plane inside of best focus and outside of best focus,respectively, thereby capturing the first and second images of the pupilimage plane of steps 1501 and 1503, respectively. In this configuration,beam splitter 353 splits the light beams produced by the lenslet array307 into at least three arms which are projected onto the multipleimaging devices (e.g., first pupil plane imaging device, second pupilplane imaging device, and Hartmann spot imaging device). Hereto, thebeam splitter 353 may comprise one or more beam splitting elements, suchas a cube-type beam splitter, plate-type beam splitter, or dichroicprism assembly as described above.

[0175] Preferably, the illumination of the reference beam and subsequentimage grabbing and processing operations that derive the locations ofthe lenslet center location for each given lenslet are performed as acalibration operation prior to wavefront sensing measurements for aneye. Moreover, such operations can be used dynamically (in a mannerconcurrent with or interposed with such wavefront sensing measurements)to perform such calibration as frequently as required to ensure theaccuracy of the system. Such dynamic operations (which may be concurrentwith or interposed with wavefront sensing measurements) enable accuratereal-time wavefront measurement while the eye moves and/or accommodates.

[0176] In addition, the calibration operations of FIG. 17 may beaccomplished with the wavefront sensing head 303″″ of FIG. 20B in amanner that avoids the delays associated with capture of the requiredmultiple images. In this configuration, the imaging device 311-A ispositioned substantially near the pupil image plane to enable the imagedevice 311-A to capture the image of the pupil image plane at best focusin step 1701.

[0177]FIGS. 20C and 20D illustrate improved Shack-Hartmann wavefrontsensing heads that embody the operations of FIG. 19 in dynamicallyidentifying sub-arrays (pixel areas) of the Hartmann spot imaging device(e.g., the imaging device that will be used for the determination ofHartmann spot positions) that avoid dot crossover for a particularwavefront measurement, while avoiding delays associated with the captureof the required multiple images. The improved wavefront sensing headsinclude a beam splitter 353 and multiple imaging devices that cooperateto capture multiple images of different planes between the lenslet arrayitself and the focal plane of the lenslet array as required by theoperations of FIG. 19. In these configurations, the beam splitter 353splits the light beams produced by the lenslet array 307 into multiplearms (preferably of equal intensity) which are projected onto themultiple imaging devices. For example, as shown, beam splitter 353splits the light beams produced by the lenslet array 307 into three arms(preferably of equal intensity) which are projected onto imaging devices311-A, 311-B1 and 311-B2. Hereto, the beam splitter 353 may comprise oneor more beam splitting elements, such as a cube-type beam splitter,plate-type beam splitter, or dichroic prism assembly as described above.In the configurations of FIGS. 20C and 20D, imaging devices 311-B1 and311-B2 capture the image of the spot image plane (test spotpattern/reference spot pattern) at best focus and inside (or outside) ofbest focus, respectively, which are used to dynamically identify thesub-arrays (pixel areas) of the Hartmann spot imaging device 311-B1(e.g., the imaging device that will be used for the determination ofHartmann spot positions) for a particular wavefront measurement.

[0178] Preferably, the image grabbing and processing operations of thereference spot image plane and the test spot image plane thatdynamically identifying sub-arrays (pixel areas) of the Hartmann spotimaging device that avoid dot crossover are performed concurrently (ornear concurrently) as part of each particular wavefront sensingmeasurement for the eye. Such concurrent (or near concurrent) operationsenable accurate real-time wavefront measurement while the eye movesand/or accommodates.

[0179] In the configuration of FIG. 20C, imaging device 311-A ispositioned to capture the fiducial point spot image formed by thelenslet array 307′, which is preferably used to identify the geometricreference to nominal null for the wavefront sensor as described above.Preferably, the illumination of the reference beam and subsequent imagegrabbing and processing operations that derive the locations of thefiducial reference spots for each given lenslet are performed as acalibration operation prior to wavefront sensing measurements for aneye. Moreover, such operations can be used dynamically (in a mannerconcurrent with or interposed with such wavefront sensing measurements)to perform such calibration as frequently as required to ensure theaccuracy of the system. Such dynamic operations (which may be concurrentwith or interposed with wavefront sensing measurements) enable accuratereal-time wavefront measurement while the eye moves and/or accommodates.

[0180] In the configuration of FIG. 20D, imaging device 311-A is mountedon a linear actuator that provides sufficient travel to allow theimaging device 311-A to capture multiple images of the pupil imageplane, which are used to i) generate the geometric reference of nominalnull for the wavefront sensor as described above with respect to FIG.15, and ii) perform the dynamic calibration operations as describedabove with respect to FIG. 17. Preferably, the illumination of thereference beam and subsequent image grabbing and processing operationsthat derive the locations of the lenslet center location for each givenlenslet are performed as a calibration operation prior to wavefrontsensing measurements for an eye. Moreover, such operations can be useddynamically (in a manner concurrent with or interposed with suchwavefront sensing measurements) to perform such calibration asfrequently as required to ensure the accuracy of the system. Suchdynamic operations (which may be concurrent with or interposed withwavefront sensing measurements) enable accurate real-time wavefrontmeasurement while the eye moves and/or accommodates.

[0181] Advantageously, all of these configurations avoid delaysassociated with capture of the required multiple images and greatlyimproves the ease of measurement of ocular aberrations. Morespecifically, such configurations enable the wavefront sensor to monitorthe high order aberrations of the eye while the eye moves, therebyavoiding the requirement that eye be physically immobilized and greatlyimproving the comfort of the patient when measuring the aberrations ofthe eye.

[0182] Moreover, such configurations enable the wavefront sensor tomonitor the high order aberrations of the eye while the eye changesfocus (i.e., accommodation occurs). As described above. this isimportant in measuring the aberrations of the eyes of children at anearly age (for example, early age screening for vision impairments suchas ambiopia. In such circumstances, drugs that paralyze accommodation(commonly called Cycloplegics) are typically used to measure theaberrations of the eye. Such drugs typically require a long time to takeeffect (30-60 minutes). In the event that the required time period forinducement of paralysis is not satisfied, prior art ophthalmicinstruments are error prone in measuring the aberration of the eye andthus susceptible to misdiagnosis. In addition, such drugs typicallyrequire a long wear off (up to 24 hours), which leads to patientdiscomfort over this prolonged period. By avoiding the use of suchdrugs, the ophthalmic instrument of the present invention avoids theseproblems, thereby minimizing the delays and inconveniences in examiningand treating such patients and enabling more accurate and efficientocular measurements and diagnosis.

[0183] In addition, such configurations enable active (e.g., dynamic)mapping of the local tilt measurements to the pupil of the eye, whichsignificantly improves the accuracy of the measurements performed by thewavefront sensor and the accuracy of the resulting wavefront dataproduced by the ophthalmic instrument.

[0184] Spot Location Utilizing Spot Fitting

[0185] The Shack-Hartmann wavefront sensors discussed above are a classof Hartmann wavefront sensors. A Hartmann wavefront sensor includes oneor subapertures that spatially sample incident light, one or moreoptical elements (such as refractive lens, diffractive grating ordiffractive hologram) that focus the samples to spots, and a mechanismfor measuring location of the spots. Exemplary Hartmann wavefrontsensors are illustrated in FIGS. 21A-21C.

[0186] The Hartmann wavefront sensor of FIG. 21A includes an apertureplate having a plurality of subapertures each sampling different spatialparts of an incident light beam and corresponding lens elements thatfocus the samples to spots. The location of each spot is measured by aspot position detector.

[0187] The Hartmann wavefront sensor of FIG. 21B includes a lens thatfocuses incident light to one or more spots at a focal point, and a spotposition detector placed at that focal point. A scanning aperture drumis located between the lens and the spot position detector. The drumincludes a plurality of subapertures that are offset from one another tothereby spatially sample the incident light directed thereto as the drumis rotated (i.e., each subaperture samples a vertical scan line of thefootprint of incident light on the rotating drum). The spot positiondetector measures the location of the spot that is formed from thesample of light that passes through each subaperture of the rotatingdrum.

[0188] The Hartmann wavefront sensor of FIG. 21C includes a monolithiclenslet array having a plurality of subapertures (lenslets) eachsampling different spatial parts of an incident light beam and focusingthe samples to spots. The location of each spot is measured by a spotposition detector.

[0189] The basic measurement performed by any Hartmann wavefront sensoris the determination of the locations of the Hartmann spots.Traditionally, this has been done by calculating the centroid of theillumination in a pixel subaperture defined around each spot. It can beshown that the position of the centroid of light is directly related tothe average tilt of the wavefront over the pixel subaperture.Unfortunately, as described above in detail in the Background ofInvention, centroid calculation has a number of difficulties (includinga high threshold signal-to-noise ratio) that limit its usefulness inmany real-world applications.

[0190] The performance of such traditional Hartmann wavefront sensorscan be enhanced through the use of an improved technique for determiningthe location of the Hartmann spot in a given pixel subaperture definedaround that spot. The improved technique, which is preferably executedby an image sensor and image processing device embodied within theHartmann wavefront sensor, is described below with reference to the flowchart of FIG. 22.

[0191] In step 2201, an image sensor captures image data (pixelintensity values) of an area defined around a Hartmann spot (e.g., pixelsubaperture) and provides the image data to the image processor.

[0192] In step 2203, the image processor exams the intensity values ofthe pixels of this pixel subaperture to roughly locate the peak of thespot. This may be accomplished simply by finding the pixel (of the pixelsubaperture) with the maximum intensity value.

[0193] In step 2205, the image processor identifies a subarray of pixelsaround the peak located in step 2203. A typical size for this sub-arrayis five by five (5×5) pixels, but it can be adjusted depending upon theparameters of the wavefront sensor.

[0194] In step 2207, the image processor performs a linear,least-squares fit on the intensity values of the subarray identified instep 2105. A function that approximates the shape of the peak of thespot is used for the fit. An example of such a function is a parabolicfunction of the form:

I=Ax ² +Bxy+Cy ² +Dx+Ey+F,

[0195] where I is the intensity signal in the extracted region and x andy are the pixel coordinates.

[0196] Finally, in step 2109, the image processor calculates the maximumof the fitted function (in pixel space). This may be accomplished bysolving the simultaneous equations γI/γx=0, γI/γy=0, or by solving theequivalent equations 2Ax_(p)+By_(p)+D=0, Bx_(p)+2Cy_(p)+E=0, where x_(p)and y_(p) are the x,y coordinates of the peak of the fit. This maximumis the estimate of the location of the spot.

[0197] Because this technique can be controlled to ignore pixels farfrom the spot, it is much less sensitive to errors in backgroundsubtraction as described above. Furthermore, only the pixels in thesubarray contribute noise to the measurement. Both of these factorscontribute to an improved signal-to-noise ratio.

[0198] According to the present invention, a Hartmann wavefront sensor(including any of the Shack-Hartmann wavefront sensor configurationsdescribed above) utilizing this improved technique is embodied within anophthalmic instrument to measure the aberrations of the human eye. Suchimprovements are broadly applicable to (and can be embodied within)ophthalmic instruments that are used to examine or treat the eye,including ophthalmic examination instruments (such as phoropters andautorefractors) that measure and characterize the aberrations of thehuman eye in addition to ophthalmic imaging instruments (such as funduscameras, corneal topographers, retinal topographers, corneal imagingdevices, and retinal imaging devices) that capture images of the eye.

[0199] The improvements provide an ophthalmic instrument withsignificant advantages. More specifically, the light level of theretinal reflections returning from the eye is typically quite low due tothe following constraints: the retina is not very reflective, and thebrightness of the wavefront sensing illumination source cannot be raisedwithout limit because of eye safety concerns and by the desire forsubject comfort. In addition, background illumination (e.g., noise) isalmost certainly present (either from scattering in the system and thesensor or from room light). In such an environment, background noiserepresents a problem. Advantageously, the robustness of the techniquedescribed above for determination of the locations of the Hartmann spotsprovides an improved signal-to-noise ratio that enables high qualitywavefront measurements of the eye under a wider range of operatingconditions (e.g., in noisier environments).

[0200] Extended Source

[0201] In another aspect of the present invention, improved Hartmannwavefront sensing mechanisms (and improved ophthalmic instrumentsutilizing these mechanisms) are provided that utilize an extended sourceto improve the signal-to-noise ratio of the wavefront measurementscalculated therein.

[0202] Note that Hartmann wavefront sensing mechanisms described aboveall share the same basic configuration—a small spot is projected ontothe retina and retro-reflected light that emerges from the eye isdirected to a Hartmann wavefront sensor that measures the phaseaberrations in the retro-reflected light directed thereto. If the spotis small enough to act as a point source, that phase aberrationsmeasured by the Hartmann wavefront sensor is representative of theaberrations of the eye.

[0203] As discussed above, the light level of the retinal reflectionsreturning from the eye is typically quite low due to the followingconstraints: the retina is not very reflective, and the brightness ofthe wavefront sensing illumination source cannot be raised without limitbecause of eye safety concerns and by the desire for subject comfort. Inaddition, background illumination (e.g., noise) is almost certainlypresent either from scattering in the system and the sensor or from roomlight. In such an environment, background noise represents a problem.

[0204]FIG. 25A illustrates a typical Hartmann wavefront sensor 2501 inwhich an aberration-inducing medium 2502 is disposed between a pointsource 2503 and the sensor 2501. The sensor 2501 includes foreoptics2506 and a plurality of subapertures 2504 (e.g., lens array) thatre-image the point source 2503 to form the Hartmann spot pattern 2505.Foreoptics 2506 is drawn as a refractive type element. However, it iswell known to those skilled in the optical engineering art that suchforeoptics 2506 can include one or more reflective, refractive ordiffractive type elements. Each subaperture samples a small portion ofthe full input pupil of the sensor 2501. An imaging device 2507 capturesimages of the Hartmann spot pattern 2505 formed by the subapertures 2503and outputs image data representative of the Hartmann spot pattern. Animage processing computer 2509 generates an estimate of the gradientfield of the input wavefront by analyzing the image data to derive ameasure of the locations of the centroids of the spots in the Hartmannspot pattern. The location of the centroid of a given spot is simplyrelated to the tilt of the wavefront over the subaperture that forms thegiven spot.

[0205]FIG. 25B illustrates an improved Hartmann wavefront sensor 2511 inwhich an aberration-inducing medium 2512 is disposed between an extendedsource (an illumination source of arbitrary dimensions) 2513 and thesensor 2511. The sensor includes foreoptics 2516 and a plurality ofsubapertures 2514 (e.g., lens array) that form a plurality of images2515 of the extended source 2513. Foreoptics 2516 is drawn as arefractive type element. However, it is well known to those skilled inthe optical engineering art that such foreoptics 2516 can include one ormore reflective, refractive or diffractive type elements. Eachsubaperture samples a small portion of the full input pupil of thesensor 2511. An imaging device 2517 (e.g., one or more CCD-based orCMOS-based image sensors) capture the plurality of images 2515 of theextended source 2513 formed by the subapertures 2513 and outputs imagedata representing such images 2515. An image processing computer 2519generates an estimate of the gradient field of the input wavefront byapplying image processing techniques to the image data representing suchimages 2515.

[0206] Importantly, the dimensions of the extended source 2513 can belarger that a diffraction limited spot, yet must be small enough so thatdifferent parts of the image do not experience substantially differentaberrations while passing through the aberration-inducing medium 2512.In addition, as is evident from FIG. 25B, the angular size of theextended source 2513 must be limited so that the images 2515 formed byeach subaperture (e.g., lenslet) do not overlap, which would lead toconfusion in the image plane. In general, this may be accomplishedthrough the use of an aperture stop (not shown) at the input image planeof the sensor 2511 that limits the angular field of view.

[0207] According to the present invention, the improved Hartmann sensor2511 of FIG. 25B is embodied with an ophthalmic instrument to measurethe aberrations of the eye. Importantly, the use of the extended sourceas the wavefront sensing illumination source and the formation of thevirtual extended source on the retina spreads light over a larger regionof the retina (than the prior art approach as described above thatproduced a spot image of the wavefront sensing illumination source onthe retina), which allows for the use of greater total optical powerwhile keeping the power density the same as in this prior art approach.The resulting use of greater total optical power improves thesignal-to-noise ratio of the ophthalmic instrument, thereby enablinghigh quality wavefront measurements of the eye under a wider range ofoperating conditions (e.g., in noisier environments).

[0208] When using the extended source 2513, it is necessary that theimage processing computer 2519 of the wavefront sensor 2511 utilize amore sophisticated image processing algorithm to estimate thesubaperture tilt of the incident wavefronts (which are derived fromretinal reflections of the extended source 2513). Preferably, imagecorrelation techniques in the digital domain are applied to image datathat represents the plurality of images 2515 of the extended source 2513(which is output by the imaging device 2517) to derive a correlationproduct for a given image 2515 _(i) and corresponding subaperture 2514_(j). The peak correlation point of the correlation product for thegiven image 2515 _(i)/subaperture 2514 _(j) provides the tilt estimateof the incident wavefront over the given subaperture 2514 _(j).

[0209] The correlation of two images I₁ and I₂ in the digital domain maybe generally represented as the product:$\sum\limits_{p_{1} \in \quad w_{1}}{\sum\limits_{p_{2} \in \quad w_{2}}{p_{1} \otimes p_{2}}}$

[0210] where p₁ is the pixel index running over the domain of interestw₁ in the image I₁, and p₂ is the pixel index running over the domain ofinterest w₂ in the image I₂.

[0211] The correlation product (denoted by {circle over (×)}) can bedefined by many different functions. For example, the correlationproduct can be defined by the sum of squared differences function asfollows:${p_{1} \otimes p_{2}} = {\sum\limits_{w_{1},w_{2}}( {p_{1} - p_{2}} )^{2}}$

[0212] In this example, the correlation product describes a measure ofdifference between the two interest regions of the images and representsthe quantity to be minimized to find the peak correlation point. In asecond example, the correlation product can be defined by the sum ofproducts function as follows:${p_{1} \otimes p_{2}} = {\sum\limits_{w_{1},w_{2}}( {p_{1} - p_{2}} )^{2}}$

[0213] In this second example, the correlation product describes ameasure of resemblance between the two interest regions of the imagesand represents the quantity to be maximized to find the peak correlationpoint. Other correlation products that can be used differ from these twoby the fact that they include scale and possible offset normalization inluminance.

[0214]FIG. 26 is a schematic illustration of an exemplary imagecorrelation algorithm, which can be embodied by the image processingcomputer 2519, to derive a correlation product for a given subapertureimage 2515 _(i). The peak correlation point of the correlation productfor the given image 2515 _(i) provides the tilt estimate of the incidentwavefront over the corresponding subaperture 2514 _(j) that forms thegiven image 2515 _(i).

[0215] As shown, the correlation product for a given image 2515 _(i) isderived from the image data (pixel intensity values) of an area definedaround the given image 2515 _(I), which is provided by the imagingdevice 2517 and denoted “subaperture image data” for the sake ofdescription. In addition, the correlation product for a given image 2515_(i) is derived from reference image data. The reference image data maybased upon: the collection of image data (over time) for given image2515 _(i); a collection of image data for a plurality of subapertures,for example, comprising the average subaperture image (wherein eachpixel in the average subaperture image represents the average intensityvalue for that pixel over of all of the subaperture image data); or areference source, for example, formed by projecting a perfect (orsubstantially non-aberrated) image of the extended source 2513 (or likesource) onto the plane of the subapertures 2514 and capturing the imagesformed by the subapertures 2514 in response to this reference.

[0216] In block 2601, a two-dimensional Fourier transform of thesubaperture image data is calculated. 1507, the image processorcalculates the real and imaginary parts of a two-dimensional Fouriertransform of the third composite image generated in step 1505. Thetwo-dimensional Fourier transform can generally be represented asfollows:${F( {k,l} )} = {\frac{1}{N\quad M}*{\sum\limits_{x = 0}^{N - 1}{\sum\limits_{y = 0}^{M - 1}{{f( {x,y} )}^{- {{2\Pi}{({\frac{k\quad x}{N} + \frac{l\quad y}{M}})}}}}}}}$

[0217] where N represents the number of pixels in each row (i.e., xdirection) of the image data;

[0218] M represents the number of pixels in a column (i.e., y direction)of the image data;

[0219] f(x,y) represents the intensity value at a pixel (x,y) in theimage data; and

[0220] the exponential term is the basis function corresponding to eachpoint F(k,l) in Fourier space.

[0221] This transform can be calculated as a double sum at each imagepoint as follows:${F( {k,l} )} = {\frac{1}{M}*{\sum\limits_{y = 0}^{M - 1}{{P( {k,y} )}^{{- {2\Pi}}\frac{ly}{M}}}}}$w  h  e  r  e${P( {k,y} )} = {\frac{1}{N}*{\sum\limits_{x = 0}^{N - 1}{{f( {x,y} )}^{{- {2\Pi}}\frac{kx}{N}}}}}$

[0222] In block 2603, a two-dimensional Fourier transform of thereference image data is calculated. These operations are similar to theoperations described above for the subaperture image data.

[0223] In block 2605, the complex conjugate function is applied to theresultant Fourier transform data calculated in block 2603. As is wellknown, the complex conjugate function operates to flip the sign of theimaginary part of the Fourier transform data calculated in block 2603.

[0224] In block 2607, the Fourier transform data calculated in block2601 is multiplied by the resultant data calculated in block 2605.

[0225] In block 2609, a two-dimensional inverse Fourier transform of theresultant data of block 2607 is calculated. The result of thetwo-dimensional inverse Fourier transform is the correlation product forthe given subaperture image 2515 _(i). The two-dimensional inverseFourier transform can be generally represented as follows:${f( {x,y} )} = {\frac{1}{N\quad M}*{\sum\limits_{k = 0}^{N - 1}{\sum\limits_{l = 0}^{M - 1}{{F( {k,l} )}^{{2\Pi}{({\frac{k\quad x}{N} + \frac{l\quad y}{M}})}}}}}}$

[0226] where N represents the number of pixels in each row (i.e., xdirection) of the image data;

[0227] M represents the number of pixels in a column (i.e., y direction)of the image data;

[0228] f(x,y) represents the intensity value at a pixel (x,y) in theimage data; and

[0229] the exponential term is the basis function corresponding to eachpoint F(k,l) in Fourier space.

[0230] Finally, in block 2611, the peak correlation point (i.e., themaximum quantity in this example) of the correlation product generatedin block 2609 is identified. This peak provides the tilt estimate of theincident wavefront over the corresponding subaperture 2514 _(j) thatforms the given subaperture image 2515 _(i).

[0231] These local tilt estimates can then be reconstructed to form datarepresentative of the aberrations (including defocus, sphericalaberration, coma, astigmatism in addition to other higher orderaberrations) of the distorted wavefront incident on the subapertures2514. For example, the local tilt estimates may be reconstructed into anoptical path difference (OPD) array, which stores a scalar value thatrepresents the optical path difference at each subaperture.Alternatively, the local tilt estimates may be reconstructed into an OPDfunction, for example, by minimizing the difference between thederivatives of an analytical function (such as a set of Zernikepolynomials, Seidel polynomials, Hermites polynomials, Chebychevpolynomials, and Legendre polynomials) and the measured local tiltestimates.

[0232] In addition, the data representative of the aberrations(including defocus, spherical aberration, coma, astigmatism in additionto other higher order aberrations) of the distorted wavefront incidenton the subapertures 2514 is preferably used to generate a graphicalrepresentation (such as a wavefront map that depicts the OPD over thepupil, e.g., subapertures, of the wavefront sensor 2511′, or a graphicaldisplay of the coefficients of the OPD function as illustrated in FIG.6C) of the aberrations of the eye 1.

[0233] Note that the correlation operations of FIG. 26 are primarilycarried out by applying the multiplication of a complex conjugate in thefrequency domain. Carrying out the correlation operations in thefrequency domain provides inherent computational efficiency for largeimages. However, one skilled in the image processing arts will realizethat these operations are analogous to computing the correlation as asum of products of the subaperture image data and the reference imagedata in the spatial domain. In some cases (e.g., where the subapertureimage data and the reference image data are not large), it may beadvantageous to perform the correlation operations in the spatialdomain.

[0234] The improved Hartmann wavefront sensor 2511 of FIG. 25B wasdemonstrated in the laboratory. The extended source 2513 comprisedseveral handwritten characters on a piece of paper illuminated with atungsten lamp. This extended source 2513 was placed about 3 meters awayfrom the foreoptics 2513 that supplies the collimated input to thesubapertures 1514 (e.g., lenslet array) of the sensor 2511. Theaberration inducing medium 2512 was simulated wherein defocus of theimages of the extended source was induced by varying the focal power ofthe foreoptics 2513 by a predetermined amount and measuring thewavefront. The measured focal change agreed to better than {fraction(1/10)} wave with the predetermined change in focus.

[0235] As described above, the improved Hartmann sensor 2511 is broadlyapplicable to (and can be embodied within) ophthalmic instruments thatare used to examine or treat the eye, including ophthalmic examinationinstruments (such as phoropters and autorefractors) that measure andcharacterize the aberrations of the human eye in addition to ophthalmicimaging instruments (such as fundus cameras, corneal topographers,retinal topographers, corneal imaging devices, and retinal imagingdevices) that capture images of the eye.

[0236] Referring now to FIG. 27A, there is shown, in schematic form, anexemplary ophthalmic instrument according to the present invention,which embodies the improved wavefront sensor of FIG. 25B. As shown, theophthalmic instrument 2701 includes an extended source 2513′ for use asthe wavefront sensing illumination source. The optical train of theinstrument forms an image of the extended source 2513′ (which for thesake of description is denoted “virtual extended source”) on the retinaof the eye 1, which is reflected (and exits the pupil of the eye asdistorted wavefronts) and directed back to the instrument 2701. Thelight produced from the extended source 2513′ forms substantially planar(e.g., phase-aligned) wavefronts that are directed to the pupil of theeye 1. These wavefronts are imaged onto the retina of the eye 1 by thecrystalline lens thereby forming the virtual extended source. Theoptical aberrations of the eye 1, such as defocus (which may be myopia(near-sightedness) or hyperopia (farsightedness)) and astigmatism aswell has many other higher order optical aberrations, cause the lightreflected from the retina of the eye 1 (including reflections of thevirtual extended source) to form distorted wavefronts at the pupil ofthe eye 1 as it leaves the eye 1.

[0237] The optical train of the instrument 2701 creates an image ofthese distorted wavefronts (which are derived from retinal reflectionsof the virtual extended source) on a phase compensator 2709, whichspatially modulates the phase of the image of the distorted wavefrontsincident thereon to produce a compensated image of such distortedwavefronts. This compensated image is recreated at the wavefront sensor2511′.

[0238] The wavefront sensor 2511′ measures the phase aberrations in thedistorted wavefronts incident therein (which are derived from retinalreflections of the extended source 2513′) and operates in a closed-loopfashion with a controller 2717 to control the phase compensator 2709(which preferably comprises a variable focus lens and deformable mirroras described below) to compensate for such phase aberrations to restorethe distorted wavefronts to phase-aligned wavefronts, which are directedto the wavefront sensor 2511′ (for further wavefront measurement andcompensation if required).

[0239] In addition, the wavefront sensor 2511′ is preferably operablycoupled (for example, via I/O interface 2720 as shown) to a displaydevice 2721 that generates a graphical representation (such as awavefront map that depicts the OPD over the pupil, e.g., subapertures,of the wavefront sensor 2511′, or a graphical display of thecoefficients of the OPD function as illustrated in FIG. 6C) of theaberrations of the eye 1 as measured by the wavefront sensor 2511′.

[0240] As shown in FIG. 27A, the optical train of the instrument 2701preferably include lens 2702, beam splitter 2703, polarizing beamsplitter 2705 and relay lens pair 2707 that cooperate to form thevirtual extended source (i.e., the image of the extended source 2513′ onthe retina of the eye 1), which is reflected and exits the pupil of theeye as distorted wavefronts and then directed back to the instrument.The relay lens pair 2707, polarizing beam splitter 2705 and beam foldingmirror 2707 create an image of these distorted wavefronts at phasecompensator 2709. The phase compensator 2709, under control ofcontroller 2717, operates to spatially modulate the phase of the imageof the distorted wavefronts incident thereon to produce a compensatedimage of such distorted wavefronts that compensate for the aberrationsof the eye under examination. This compensated image is recreated at thewavefront sensor 2511′ for wavefront sensing.

[0241] As shown in FIG. 27A, the wavefront sensor 2511′ includes beamcombiner 2711, foreoptics 2516′ and a plurality of subapertures 2514′(e.g., lens array) that cooperate to form a plurality of images 2515′ ofthe virtual extended source (i.e., the image of the extended source2513′ formed on the retina of the eye 1). Relay lens 2713 and imagingdevice 2517′ (e.g., one or more CCD-based or CMOS-based image sensors)cooperate to capture the images 2515′. The imaging device 2517′ outputsimage data representing such images 2515′. An image processing computer2519′ generates an estimate of the gradient field of the compensatedimage of the distorted wavefronts provided by the phase compensator 2709by applying image processing techniques to the image data representingimages 2515′. More specifically, the image processing computer 2519′applies image correlation techniques in the digital domain to such imagedata to derive a correlation product for a given image 2515 _(i) andcorresponding subaperture 2514 _(j). The peak correlation point of thecorrelation product for the given image 2515 _(I)/subaperture 2514 _(j)provides the tilt estimate of the incident wavefront (i.e., thecompensated image of the distorted wavefront provided by the phasecompensator 2709, which is derived from retinal reflections of theextended source 2513′) over the given subaperture 2514 _(j).

[0242] As described above, the correlation product for a given image2515 _(i) may be derived from reference image data. The reference imagedata may based upon the collection of image data for a plurality ofsubapertures, for example, comprising the average subaperture image(wherein each pixel in the average subaperture image represents theaverage intensity value for that pixel over all of the subaperture imagedata). Alternatively, the reference image data may be based upon imagedata derived from a reference source, for example, formed by projectinga perfect (or substantially non-aberrated) image of the extended source2513′ (or like source) onto the plane of the subapertures 2514′ andcapturing the images formed by the subapertures 2514′ in response tothis reference. As shown in FIG. 27A, beam splitter 2703, beam foldingmirror 2715 and beam combiner 2711 provide this functionality byprojecting a perfect (or substantially non-aberrated) image of theextended source 2513′ onto the plane of the subapertures 2514′. Inembodiments wherein such functionality is not required, these elementsmay be omitted.

[0243] The local tilt estimates generated by the image processingcomputer 2519′ can be reconstructed to form data representative of theaberrations (including defocus, spherical aberration, coma, astigmatismin addition to other higher order aberrations) of the incident wavefront(i.e., the compensated image of the distorted wavefront provided by thephase compensator 2709, which is derived from retinal reflections of theextended source 2513′) on the subapertures 2514′. For example, the localtilt estimates may be reconstructed into an optical path difference(OPD) array, which stores a scalar value that represents the opticalpath difference at each subaperture. Alternatively, the local tiltestimates may be reconstructed into an OPD function, for example, byminimizing the difference between the derivatives of an analyticalfunction (such as a set of Zernike polynomials, Seidel polynomials,Hermites polynomials, Chebychev polynomials, and Legendre polynomials)and the measured local tilt estimates.

[0244] The image processing computer 2519′ is preferably operablycoupled (for example via I/O interface 1720) to a display device 2721that generates a graphical representation (such as a wavefront map thatdepicts the OPD over the pupil, e.g., subapertures, of the wavefrontsensor 2511′, or a graphical display of the coefficients of the OPDfunction as illustrated in FIG. 6C) of the aberrations of the eye 1based upon the data representative of the aberrations (includingdefocus, spherical aberration, coma, astigmatism in addition to otherhigher order aberrations) of the incident wavefront as constructed bythe image processing computer 2519′.

[0245] The dimensions of the image of the extended source 2513′ formedon the retina of the human eye (i.e., the virtual extended source) canbe larger that a diffraction limited spot, yet must be small enough sothat different parts of the image do not experience substantiallydifferent aberrations while passing through the eye 1 (which is theaberration-inducing medium in this application). In addition, as isevident from FIG. 25B, there the angular size of the image of theextended source 2513′ must be limited so that the images 2515′ formed byeach subaperture (e.g., lenslet) do not overlap, which would lead toconfusion in the image plane. In general, this may be accomplishedthrough the use of an aperture stop (not shown) at the input image planeof the sensor 2511′ that limits the angular field of view.

[0246] The subapertures 2514′ of the sensor 2511 preferably comprise alenslet array of the type manufactured and sold by Adaptive Optics Inc,of Cambridge, Mass., assignee of the present invention, in that itcomprises a precision array of refractive microlenses formedcontinuously on a monolithic substrate. The array of microlenses arepreferably compression molded of polymethymethacrylate (PMMA) plastic,and positioned in the substrate with full edge-to-edge lenslet contactto maximize the density of lens area to total surface area (referred toas “fill factor”).

[0247] The phase compensator 2709 of the adaptive optic subsystem of theophthalmic instrument 2701 preferably comprises multiple stages (such asthe variable focus lens (VFL) and a deformable mirror as shown in FIG.7A) that compensate for different parts of the aberrations of the eye 1as estimated by the wavefront sensor 2511′. For example, suchaberrations can be decomposed into a defocus component (which representsthe defocus of the eye 1) and one or more additional components (whichrepresent the higher order components (e.g., spherical aberration,astigmatism and coma) of such aberrations. In this case, the first stage(i.e., the variable focus lens) is used to compensate for the defocuscomponent of such aberrations, and the one or more additional stages(i.e., deformable mirror) are used to compensate for the remaininghigher order components of such aberrations. A deformable mirrorachieves such compensation by warping its optical surface to form thecomplex conjugate of such higher order components as measured by thewavefront sensor 2511′. Exemplary silicon micro-machined membranedeformable mirrors are described above with respect to FIGS. 8A and 8B.

[0248] Referring now to FIG. 27B, there is shown, in schematic form, anexemplary ophthalmic instrument 50″″ according to the present invention,which embodies the improved wavefront sensor of FIG. 25B and providesthe same functionality as the instrument of FIG. 6B as described above.

[0249] Wavefront sensing is provided by an extended source 2513″ thatcooperates with lens 2702′, beam splitter 2703′, beam combiner 129′,first polarizing beam splitter/quarter wave plate 103′/105′ and firstrelay lens group LG₁ to form a virtual extended source (i.e., image ofthe extended source 2513″) on the retina of the eye 1, which isreflected (and exits the pupil of the eye as distorted wavefronts) anddirected back to the instrument 50″″. The first relay lens group LG₁,first polarizing beam splitter/quarter wave plate 103′/105′ and secondpolarizing beam splitter/quarter wave plate 109′/111′ create an image ofthe distorted wavefronts on a phase compensator 53. The phasecompensator 53 (which preferably comprises a variable focus lens anddeformable mirror as described below) operates to spatially modulate thephase of the wavefronts incident thereon to create a compensated imageof such distorted wavefronts. The second polarizing beamsplitter/quarter wave plate 109′/111′, dielectric filter 114, beamsplitter 117′ and second relay lens group LG₂ recreate the image of suchcompensated wavefronts at wavefront sensor 2511′. The dielectric filter114 operates to selectively reflect the band of light provided by thefixation target 71, while passing the band of light provided by theextended source 2513″ (and used for wavefront sensing) in addition tothe band of light provided by the imaging illumination source 97 (andused for image capture).

[0250] The wavefront sensor 2511′ measures the phase aberrations in thewavefronts incident thereon (i.e., the compensated image of thedistorted wavefront provided by the phase compensator 53, which isderived from retinal reflections of the extended source 2513′) andoperates in a closed-loop fashion with a controller 57 to control thephase compensator 53 to spatially modulate the phase of the wavefrontsincident thereon to compensate for such phase aberrations to therebyrestore the distorted wavefronts to phase-aligned wavefronts, which aredirected to the wavefront sensor 2511′ (for further wavefrontmeasurement and compensation if required).

[0251] The wavefront sensor 2511′ is preferably operably coupled (forexample, via I/O interface 121) to a display device 123 that generates agraphical representation of the aberrations of the eye 1 as measured bythe wavefront sensor 2511′. For example, the graphical representation ofthe aberrations of the eye 1 displayed by the display device 123 may bea wavefront map that depicts the OPD over the pupil, e.g., subapertures,of the wavefront sensor 2511′, or a graphical display of thecoefficients of the OPD function as illustrated in FIG. 6C.

[0252] The fixation target is provided by an internal fixation target 71(e.g., a visible image source) that cooperates with a third relay lensgroup LG₃, dielectric filter 114, and second polarizing beamsplitter/quarter wave plate 109′/111′ to create an image of a fixationtarget 71 at the phase compensator 53. The phase compensator 53, undercontrol of controller 57, operates to spatially modulate the phase ofthe image of the fixation target to compensate for the aberrations ofthe eye under examination as measured by the wavefront sensor 55. Thesecond polarizing beam splitter/quarter wave plate 109′/111′, firstpolarizing beam splitter/quarter wave plate 103′/105,′ and first lensgroup LG₁ recreate the phase compensated image of the fixation target 71produced by the phase compensator 53 at the pupil of the eye 1 underexamination. This operation provides the patient with a view ofcorrection (e.g., compensation) of the aberrations of the eye 1 underexamination such the patient can provide instant feedback as to theaccuracy of the measurement.

[0253] Image capture is provided by an imaging illumination source 87(e.g., halogen or xenon flash lamp) that cooperates with lens 127, beamcombiner 129, first polarizing beam splitter/quarter wave plate103′/105′, and first lens group LG₁ to direct light produced from theimaging illumination source 87 onto the pupil of the eye 1, which isreflected and directed back to the instrument pupil. The first lensgroup LG_(1.), first polarizing beam splitter/quarter wave plate103′/105′, and second polarizing beam splitter/quarter wave plate109′/111′ create an image of these reflections on the phase compensator53. The phase compensator 53, under control of controller 57, operatesto spatially modulate the phase of such images to compensate for theaberrations of the eye 1 as measured by the wavefront sensor 55. Thesecond polarizing beam splitter/quarter wave plate 109′/111′, dielectricfilter 114, beam splitter 117′ and fourth relay lens group LG₄ recreatethe compensated image of such reflected wavefronts as produced by thephase compensator 53 at imaging device 89 (such as a CCD camera body,3-CCD camera body, CMOS camera body and/or a photographic film unit) forcapture. This operation provides the user with the capability ofacquiring high resolution images of the eye 1.

[0254] As is well known in the art, spectral filters that are tuned tothe wavelength of the extended source 2513″ and/or imaging illuminationsource 87 may be disposed along the optical path between the beamsplitter 117′ and the wavefront sensor 55 and imaging device 89,respectively, in order to reduce background noise and noise from theother illumination sources of the instrument.

[0255] As shown in FIG. 27A, the wavefront sensor 2511′ includes beamcombiner 2711, collimating lens 2516′ and a plurality of subapertures2514′ (e.g., lens array) that cooperate to form a plurality of images2515′ of the extended source 2513″, which are derived from retinalreflections of the extended source 2513″. A relay lens 2713 and imagingdevice 2517′ (e.g., one or more CCD-based or CMOS-based image sensors)cooperate to capture the plurality of images 2515′. The imaging device2517′ outputs image data representing such images 2515′. An imageprocessing computer 2519′ generates an estimate of the gradient field ofthe incident wavefront (i.e., the compensated image of the distortedwavefront provided by the phase compensator 53, which is derived fromretinal reflections of the extended source 2513″) by applying imageprocessing techniques to the image data representing images 2515′. Morespecifically, the image processing computer 2519′ applies imagecorrelation techniques in the digital domain to such image data toderive a correlation product for a given image 2515 _(i) andcorresponding subaperture 2514 _(j). The peak correlation point of thecorrelation product for the given image 2515 _(i)/subaperture 2514 _(j)provides the tilt estimate of the incident wavefront (i.e., thecompensated image of the distorted wavefront provided by the phasecompensator 53, which is derived from retinal reflections of theextended source 2513″) over the given subaperture 2514 _(j).

[0256] As described above, the correlation product for a given image2515 _(i) may be derived from reference image data. The reference imagedata may based upon the collection of image data for a plurality ofsubapertures, for example, comprising the average subaperture image(wherein each pixel in the average subaperture image represents theaverage intensity value for that pixel over all of the subaperture imagedata). Alternatively, the reference image data may be based upon imagedata derived from a reference source, for example, formed by projectinga perfect (or substantially non-aberrated) image of the extended source2513′ (or like source) onto the plane of the subapertures 2514′ andcapturing the images formed by the subapertures 2514′ in response tothis reference. As shown in FIG. 27B, beam splitter 2703′, beam foldingmirrors 2715 ₁′ and 2715 ₂′ and beam combiner 2711 provide thisfunctionality by projecting a perfect (or substantially non-aberrated)image of the extended source 2513″ onto the plane of the subapertures2514′. In embodiments wherein such functionality is not required, theseelements may be omitted.

[0257] The local tilt estimates generated by the image processingcomputer 2519′ can be reconstructed to form data representative of theaberrations (including defocus, spherical aberration, coma, astigmatismin addition to other higher order aberrations) of the incident wavefront(i.e., the compensated image of the distorted wavefront provided by thephase compensator 53, which is derived from retinal reflections of theextended source 2513″) on the subapertures 2514′. For example, the localtilt estimates may be reconstructed into an optical path difference(OPD) array, which stores a scalar value that represents the opticalpath difference at each subaperture. Alternatively, the local tiltestimates may be reconstructed into an OPD function, for example, byminimizing the difference between the derivatives of an analyticalfunction (such as a set of Zernike polynomials, Seidel polynomials,Hermites polynomials, Chebychev polynomials, and Legendre polynomials)and the measured local tilt estimates.

[0258] The image processing computer 2519′ is preferably operablycoupled (for example via I/O interface 121) to a display device 123 thatgenerates a graphical representation (such as a wavefront map thatdepicts the OPD over the pupil, e.g., subapertures, of the wavefrontsensor 2511′, or a graphical display of the coefficients of the OPDfunction as illustrated in FIG. 6C) of the aberrations of the eye 1based upon the data representative of the aberrations (includingdefocus, spherical aberration, coma, astigmatism in addition to otherhigher order aberrations) of the incident wavefront as constructed bythe image processing computer 2519′.

[0259] The dimensions of the image of the extended source 2513″ formedon the retina of the human eye can be larger that a diffraction limitedspot, yet must be small enough so that different parts of the image donot experience substantially different aberrations while passing throughthe eye 1 (which is the aberration-inducing medium in this application).In addition, as is evident from FIG. 25B, there the angular size of theimage of the extended source 2513″ must be limited so that the images2515′ formed by each subaperture (e.g., lenslet) do not overlap, whichwould lead to confusion in the image plane. In general, this may beaccomplished through the use of an aperture stop (not shown) at theinput image plane of the sensor 2511′ that limits the angular field ofview.

[0260] The phase compensator 53 of the adaptive optic subsystem of theophthalmic instrument 50″″ preferably comprises multiple stages (such asthe variable focus lens (VFL) and a deformable mirror as shown in FIG.7A) that compensate for different parts of the aberrations of the eye 1as estimated by the wavefront sensor 2511′. For example, suchaberrations can be decomposed into a defocus component (which representsthe defocus of the eye 1) and one or more additional components (whichrepresent the higher order components (e.g., spherical aberration,astigmatism and coma) of such aberrations. In this case, the first stage(i.e., the variable focus lens) is used to compensate for the defocuscomponent of such aberrations, and the one or more additional stages(i.e., deformable mirror) are used to compensate for the remaininghigher order components of such aberrations. A deformable mirrorachieves such compensation by warping its optical surface to form thecomplex conjugate of such higher order components as measured by thewavefront sensor 2511′. Exemplary silicon micro-machined membranedeformable mirrors are described above with respect to FIGS. 8A and 8B.

[0261] Importantly, the use of the extended source as the wavefrontsensing illumination source and the formation of the virtual extendedsource on the retina as described above spreads light over a largerregion of the retina (than the prior art approach as described abovethat produced a spot image of the wavefront sensing illumination sourceon the retina), which allows for the use of greater total optical powerthan the prior art approach. The resulting use of greater total opticalpower improves the signal-to-noise ratio of the ophthalmic instrument,thereby enabling high quality wavefront measurements of the eye under awider range of operating conditions (e.g., in noisier environments).

[0262] More specifically, if the eye is illuminated with a collimatedbeam to produce a point of light on the retina, the angular size of thatspot will be about equal to the resolution limit of the eye. For typicalvision, that limit is about 1 minute of arc or 0.3 milliradians.Conservatively, virtually all of the light in the point source will fallwithin a 1 milliradian spot on the retina. If, instead, an extendedsource is used, the light may be spread over a larger region of theretina. This allows the use of a greater total optical power whilekeeping the power density the same as in the point source case.

[0263] These advantages may be quantified as follows. Consider awavefront sensor with a spatial sampling of 7 subapertures across thepupil. Assume that a 1000×1000 pixel focal plane is used. This impliesthat each subaperture focal plane covers about 150×150 pixels. If thefocal ratio of the subapertures is chosen so that the subaperturediffraction limited spot covers 3 pixels from peak to null (a typicalrule of thumb for Hartmann sensors) then the subaperture field of viewis about 25 mrad. To prevent overlap of subaperture images a fullaperture field stop of 20 mrad could be used. This means that the sourcesize can be increased from 1 mrad to 20 mrad. At the same power densityon the retina, the total power is increased by a factor of 400.

[0264] For the point source, all the wavefront information in asubaperture is contained in the single spot of roughly diffractionlimited size. When using the extended source, image correlation can useall the information in 400 diffraction limited blur spot sized regionsto estimate the wavefront tilt. This should lead to roughly a factor of20 improvement in the tilt estimate for the same signal to noise ratioper pixel. Conversely, the light intensity could be reduced while stillachieving the same measurement accuracy as the point source case.

[0265] Ophthalmic Instrument Providing Efficient Prescriptions ofCorrective Optics

[0266] In another aspect of the present invention, the capabilities ofan ophthalmic instrument as described above (which provides the humaneye with a view of compensation of the estimated aberrations of the eyeas measured by wavefront sensing) can be exploited to enable moreeffective and efficient prescription of corrective optics (e.g., glassesor contact lens). FIG. 23 is a flow chart that illustrates theoperations of an ophthalmic instrument that exploits such capabilitiesto provide more effective and efficient prescription of correctiveoptics.

[0267] In step 2301, the ophthalmic instrument performs wavefrontsensing operations that estimate the aberrations of the eyes of thepatient.

[0268] In step 2303, the ophthalmic instrument identifies a set ofpredetermined prescriptions (which specify the correction for correctiveoptics) that correspond to the such estimated aberrations. For example,each given prescription in the set can be selected from a database ofprescriptions if the given prescription matches the estimatedaberrations within a predetermined tolerance interval.

[0269] In step 2305, for each given prescription in set identified instep 2303, the phase compensator of the ophthalmic instrument iscontrolled to generate a compensated image of the fixation target basedupon the given prescription, to enable the eyes to view the correctionspecified by the prescription.

[0270] Finally, in step 2307, feedback is collected from patient todetermine the best prescription within the set.

[0271] Advantageously, the operations of an ophthalmic instrument ofFIG. 23 enables instant feedback from the patient regarding themeasurement and correction of the aberrations of the eye, therebyproviding more effective and efficient prescription of correctiveoptics.

[0272] Ophthalmic System Providing Efficient Dispensing of CorrectiveOptics

[0273] In another aspect of the present invention, the capabilities ofan ophthalmic instrument as described above (which provides the humaneye with a view of compensation of the estimated aberrations of the eyeas measured by wavefront sensing) can be exploited to effectively enabledispensing of corrective optics (e.g., glasses or contact lens) without(or with minimal) additional human involvement beyond the patient'sinvolvement. FIGS. 24A and 24B illustrates a system (and the operationsperformed therein) that exploits such capabilities to provide dispensingof corrective optics (e.g., glasses or contact lens) without (or withminimal) additional human involvement beyond the patient's involvement.

[0274] As shown in FIG. 24A, the system includes an ophthalmicinstrument 50′ which provides the human eye with a view of compensationof the estimated aberrations of the eye as measured by wavefrontsensing. In the binocular configuration shown, the optical train of theadaptive optic based ophthalmic instrument is duplicated (e.g., twochannels, one for each eye). Any required image processing and controlmay be performed by separate devices for each channel (or suchprocessing and control may be performed on one or more shared devicesfor the channels). An imaging and dimension subsystem 501 that iscapable of generating a profile of the dimensions (and/or other relevantspatial characteristics) of the face and head of the patient may beoptionally provided. The subsystem 501 preferably sweeps a laser beamover the face and head of the patient to generate a range data map ofthe patient's face and head, and analyzes the range data map to derivethe profile. Details and alternate embodiments of the imaging anddimension subsystem 501 may be found in co-pending U.S. patentapplication Ser. No. 09/327,756 filed Jun. 7, 1999 (Attorney Docket No.108-068USA000) and International Application PCT/US00/15624, filed Jun.7, 2000 (Attorney Docket No. 108-085PCT000), all commonly assigned tothe assignee of the present invention and herein incorporated byreference in their entirety. Such a system is used to automatically fitframes to the size and shape of the patient's face and head whendispensing glasses. It is not required for the dispensing of contactlens.

[0275]FIG. 24B is a flow chart that illustrates the operations of thesystem of FIG. 24A that provides the dispensing of corrective optics(e.g., glasses or contact lens) with minimal human assistance to thepatient. For the sake of description, it is assumed that there is aninventory of corrective optics and frames which can be made available tothe patient. However, it should be readily apparent that the correctiveoptics and/or frames ultimately provided to the patent can be custombuilt according to specifications provided by the system.

[0276] In step 2401, the ophthalmic instrument 50′ performs wavefrontsensing operations that estimate the aberrations of the eyes of thepatient.

[0277] In step 2403, the ophthalmic instrument 50′ identifies a set ofcorrective optics in the inventory that correspond to the estimatederror. For example, each given corrective optic in the set can beselected from the inventory of corrective optics if the given correctiveoptic matches the estimated aberrations within a predetermined toleranceinterval.

[0278] In step 2405, for each corrective optic in set identified in step2403, the phase compensator of the ophthalmic instrument 50′ iscontrolled to generate a compensated image of the fixation target basedupon the given corrective optic, to enable the eyes to view thecorrection provided by the given corrective optic.

[0279] In step 2407, the patient selects one of corrective optics in theset (if necessary).

[0280] In step 2409, the imaging and dimension subsystem 501 optionallyperforms imaging and dimensioning analysis of the head and face of thepatient to generate a profile of the dimensions (or other relevantspatial characteristics) of the face and/or head of the patient.

[0281] In step 2411, one or more frames that correspond to the profilegenerated in step 2409 are identified (or custom built). For example,those frames that best fit the size and shape of the patient's faceand/or head can be selected.

[0282] In step 2413, the patient selects one of the frames identified instep 2411 (if necessary) Finally, in step 2415, the patient is providedwith corrective optic selected in step 2407. When the corrective opticare glasses, the patient is provided with glasses that includes theselected corrective optic and the selected (or custom built) frame thatbest fits the size and shape of the patient's face and/or head asidentified in step 2411.

[0283] Preferably, the operations of FIG. 24B are performedautomatically (without human involvement). Alternatively, minimal humaninvolvement (for example, a clerk locating the selected corrective opticfrom the inventory) may be used to dispense the corrective optic (orglasses) selected by the patient. Advantageously, the operations of thesystem of FIGS. 24A and 24B enables instant feedback from the patientregarding the measurement and correction of the aberrations of the eye.Moreover, such operations lowers the costs of dispensing the correctiveoptics (and glasses).

[0284] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as examples only, with the true scope of theinvention being indicated by the claims to Invention appended hereto.

1. An ophthalmic instrument comprising: a wavefront sensor thatestimates aberrations in reflections of the light formed as an image onthe retina of the human eye; and a phase compensator, operably coupledto the wavefront sensor, that spatially modulates the phase of incidentlight to compensate for said aberrations estimated by the wavefrontsensor, wherein said phase compensator comprises a first stage and atleast one other additional stage, wherein said first stage compensatesfor a defocus component of said aberrations, and wherein said at leastone other additional stage compensates for other higher order componentsof said aberrations.
 2. The ophthalmic instrument of claim 1, whereinsaid first stage comprises a variable focus lens.
 3. The ophthalmicinstrument of claim 2, wherein said variable focus lens comprises firstand second lens elements aligned along an optical axis, wherein saidfirst lens element is moved with respect to said second lens elementlinearly along said optical axis by a linear actuator.
 4. The ophthalmicinstrument of claim 1, wherein said at least one other additional stagecomprises a deformable mirror.
 5. The ophthalmic instrument of claim 4,wherein said deformable mirror comprises a silicon micro-machinedmembrane mirror including a silicon chip mounted over a printed circuitboard substrate by spacers, wherein a top surface of said silicon chipcomprises a membrane which is coated with a reflective layer to form amirror surface, and wherein the printed circuit board comprises acontrol electrode structure that operates to deform the shape of thereflective membrane by applying bias and control voltages to themembrane and control electrodes disposed therein.
 6. The ophthalmicinstrument of claim 1, wherein said at least one other additional stagecomprises a liquid crystal device.
 7. The ophthalmic instrument of claim1, further comprising optical elements that enable a user to examine ortreat the eye.
 8. The ophthalmic instrument of claim 1, furthercomprising a display device that displays a graphical representation ofaberrations of the eye, wherein said graphical representation is basedupon said aberrations estimated by said wavefront sensor.
 9. Anophthalmic instrument comprising: a wavefront sensor that estimatesaberrations in reflections of the light formed as an image on the retinaof the human eye; a phase compensator, operably coupled to the wavefrontsensor, that spatially modulates the phase of incident light tocompensate for said aberrations estimated by the wavefront sensor,wherein said phase compensator comprises a variable focus lens thatcompensates for a defocus component of the aberrations estimated by thewavefront sensor.
 10. The ophthalmic instrument of claim 9, wherein saidphase compensator further comprises a deformable mirror that compensatedfor other higher order components of the aberrations estimated by thewavefront sensor.
 11. The ophthalmic instrument of claim 9, wherein saidphase compensator further comprises a liquid crystal device thatcompensates for other higher order components of the aberrationsestimated by the wavefront sensor.
 12. The ophthalmic instrument ofclaim 9, further comprising optical elements that enable a user toexamine or treat the eye.
 13. The ophthalmic instrument of claim 9,further comprising a display device that displays a graphicalrepresentation of aberrations of the eye, wherein said graphicalrepresentation is based upon said aberrations estimated by saidwavefront sensor.